Catalytic ammonia synthesis by transition metal molecular complexes

This application claims the benefit of U.S. Provisional Application No. 61/891,550, filed Oct. 16, 2013, which is hereby incorporated by reference in its entirety to the extent not inconsistent herewith.

This invention was made with government support under GM070757 awarded by the National Institutes of Health. The government has certain rights in the invention.

Catalytic conversion of molecular nitrogen to ammonia is a critical component of the global nitrogen cycle. This chemical transformation is carried out via a combination of natural biological processes and anthropogenic activities. Biological generation of ammonia via nitrogen reduction occurs primarily by natural processes mediated by organisms resulting in nitrogen fixation necessary for sustaining life. Industrial processes for nitrogen reduction, on the other hand, are believed to generate ammonia on a roughly equivalent scale to the biological sources. For example, the annual industrial production of NH 3 via nitrogen reduction is estimated to be approximately 200 million tonnes per year and supports a range of commercial applications including fertilizers, decontamination and/or sterilization agents, and precursors for the production of other nitrogen containing chemicals.

Industrially, the majority of ammonia is produced in commercial quantities via the Haber-Bosch process. This process involves the reaction of nitrogen gas with hydrogen gas in the presence of a solid-state catalyst to produce ammonia. A range of catalysts have been developed for the Haber-Bosh process including iron-based catalysts and ruthenium-based catalysts. The Haber-Bosch process is resource intensive as it typically involves high pressures (˜100 atm), high temperatures (˜450° C.), as well as an industrial supply of precursor hydrogen gas. Given these requirements, ammonia production via industrial processes represents a significant amount of total energy consumed throughout the world each year (estimated to be as much as 1%).

In contrast, the transformation of nitrogen gas to ammonia by biological processes occurs efficiently under ambient conditions. This process is mediated by organisms, such as diazotrophs, and involves cofactors of nitrogenase enzymes rich in Fe and S and which may additionally feature Mo and V. Substantial research has been directed toward understand the chemical mechanism of the iron-molybdenum cofactor, for example, that provides for catalytic conversion of nitrogen into ammonia at low temperatures and pressures. Despite extensive research over the last several decades, the exact mechanism for biological reduction of nitrogen to ammonia remains uncertain.

The development and study of functionally biomimetic catalysts provide useful tools for understanding the mechanism involved in nitrogen reduction by nitrogenase enzymes. Such functionally biomimetic catalysts also provide a potential pathway for development of industrially relevant molecular catalysts providing for the reduction of N 2 to NH 3 . An example of this has been approach is the development and characterization of transition metal molecular complexes capable of binding and functionalizing N 2 under ambient conditions.

Molecular systems for reduction of N 2 to NH 3 have traditionally focused on Mo centers given the presence of Mo in the most thoroughly studied iron-molybdenum cofactor. Tri-amido amine Mo and phosphine-pincer Mo complexes, for example, have been demonstrated to provide moderate catalytic efficiencies for reduction of N 2 to NH 3 at ambient temperature and pressures. Attention has also been more recently directed to a potential role of iron as an active N 2 binding site in iron-molybdenum cofactor given the understanding that iron is the only transition metal essential to all nitrogenases. This approach is also supported by recent spectroscopic and biological data implicating iron, as opposed to molybdenum, as the active site of N 2 binding in the iron-molybdenum cofactor.

It will be appreciated from the foregoing that there is currently a need in the art for improved molecular catalysts and, methods capable of the facile conversion of nitrogen to ammonia. Specifically, molecular catalysts are needed for nitrogen reduction providing useful catalytic efficiencies and turnover under conditions less stringent than those in the conventional Haber-Bosh process.

This invention relates to molecular catalysts and chemical reactions utilizing the same, and particularly to catalysts and catalytic methods for reduction of molecular nitrogen. The molecular catalytic platform provided herein is capable of the facile reduction of molecular nitrogen under useful conditions such as room temperature or less and atmospheric pressure or less.

Provided herein are catalyst compositions, catalytic systems and methods for the reduction of molecular nitrogen. Transition metal catalysts, formations and catalytic methods of the invention, for example, exhibit useful catalytic efficiency and with good turnover ratio and reduction product yields under conditions supporting a range of industrial applications, including the production of ammonia. Catalysts of the invention include molecular complexes comprising iron and cobalt active sites supported by a phosphine ligand, such as a diphosphine or trisphosphine ligand, for example, a tris(phosphine)borane, tris(phosphinoaryl)borane, tris(phosphinoaryl)alkyl and tris(phosphinoaryl)silyl ligand. Catalytic systems and methods of the invention are versatile and can be implemented using a range of process conditions and reactants, including a variety of sources of protons and electrons. Catalytic systems and methods of the invention are compatible with approaches and reaction conditions for kinetically driven catalyst formation, activation and regeneration steps. In an embodiment, the invention provides functionally biomimetic Fe or Co metal catalysts and catalytic methods for efficient conversion of molecular nitrogen to reduction products such as, for example, ammonia (NH 3 ) and/or hydrazine (H 2 H 4 ).

In an aspect, the invention provides a catalytic process for the reduction of molecular nitrogen (N 2 ) to generate a reduction product, the process comprising the steps of: contacting a transition metal catalyst with a source of protons and a source of electrons in the presence of the molecular nitrogen, thereby generating the reduction product; wherein the transition metal catalyst comprises a metal complex comprising a transition metal atom selected from the group consisting of Fe and Co, and a phosphine ligand (L). In an embodiment, for example, the metal complex comprises a coordination complex. In an embodiment, for example, the phosphine ligand (L) is a diphosphine ligand or a trisphosphine ligand. In an embodiment, multiple atoms or functional groups of the ligand independently establish bonds to the metal atom. In an embodiment, for example, the phosphine ligand is a polydentate ligand, such as a bidentate, tridentate or tetradentate ligand. In an embodiment, for example, the metal complex further comprises an N 2 group, for example bonded to a central metal atom of the metal complex.

In an embodiment, the transition metal catalyst, a source of protons, a source of electrons and molecular nitrogen are brought in physical contact with each other, for example, in a solution phase, a liquid phase, a gas phase or a combination of these. In an embodiment, for example, the process is carried out in a mixture comprising the transition metal catalyst, the source of protons, the source of electrons and molecular nitrogen, and optionally one or more solvents and/or additives. In an embodiment, for example, the reduction product is NH 3 and/or N 2 H 4 . In an embodiment the reduction product is substantially NH 3 (e.g., 90% or greater yield and optionally 99% or greater yield) or entirely NH 3 . In an embodiment, the process occurs at a temperature less than or equal to 25° C. and/or at a pressure less than or equal to 1 atmosphere. In an embodiment, the process occurs at a temperature less than 0° C.

In an embodiment, a ligand (L) of the metal complex of the invention has the formula (FX1A), (FX1B) or (FX1C):

wherein X 1 is B, C, Si or P; each of L 1 , L 2 and L 3 is independently a substituted or unsubstituted C 1 -C 10 alkylene, C 3 -C 10 cycloalkylene, C 5 -C 10 arylene, or C 5 -C 10 heteroarylene; each of R 1 , R 2 , R 3 , R 4 , R 5 , R 6 and R 7 is independently hydrogen or a substituted or unsubstituted C 1 -C 8 alkyl, C 3 -C 8 cycloalkyl, C 5 -C 8 aryl, C 5 -C 8 heteroaryl, C 1 -C 18 acyl, C 2 -C 8 alkenyl, C 2 -C 8 alkynyl, or —P(OR 8 ) 2 , wherein each R 8 is independently H, C 1 -C 8 alkyl, C 3 -C 8 cycloalkyl, C 5 -C 8 aryl or C 5 -C 8 heteroaryl. In an embodiment, the ligand of the metal complex is neutral or is charged, such as having a charge of −1 or −2.

As will be generally understood by one having skill in the art, the ligands described herein participate in bonding with the metal atom of the metal complex, for example via electron donating interactions. Therefore, the valencies of atoms of the ligand (L) structures disclosed herein, including in formulas (FX1)-(FX10), are in some cases intended to be supplemented by the bonding of the ligand to the metal atom of the metal complexes. Bonding to the metal atom can include various atoms and functional groups of the ligand including one or more of the phosphorous atoms, central atoms (X 1 ), linking groups (L 1 , L 2 and L 3 ) and/or terminating groups (R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 and R 8 ) as shown in formulas (FX1)-(FX10). Accordingly, in some embodiments, the ligand structures and/or metal complex structures as shown herein may be considered ionic and/or multivalent structures (e.g., divalent, trivalent, tetravalent, etc.). In some embodiments, for example, a ligand of the metal complexes of the invention may be viewed as an anion, for example, an anion having a −1 or −2 charge state.

In an embodiment, for example, a metal complex of the invention further comprises N 2 and has the formula (FX2A), (FX2B) or (FX2C):

wherein Z is the transition metal atom.

An important property of the invention is the catalytic nature of the reduction process, for example, wherein the transition metal catalyst is efficiently regenerated in the process and, thus, available to subsequently react with molecular nitrogen and further generate the reduction product. In an embodiment, for example, regenerated transition metal catalyst subsequently participates in further reaction with the source of protons, source of electrons and molecular nitrogen to achieve catalytic reduction. In an embodiment, the present process is characterized by a turnover frequency greater than or equal to 1×10 −2 sec −1 . In an embodiment, the present process generates at least 1 reduction product equivalents per transition metal catalyst equivalent and for some applications generates at least 2 reduction product equivalents per transition metal catalyst equivalent.

In an embodiment, for example, the process is characterized by catalyst activation and regeneration steps that are capable of kinetic control, for example, via providing acids, reductants and/or additives to kinetically drive catalyst formation, activation, hydrogenation and/or regeneration steps. The present processes are compatible with a range of different sources of electrons and sources of protons such as acids including HBAr F 4 (hydro tetrakis[(3,5-trifluoromethyl)phenyl]borate) and reductants including Na and KC 8 . In an embodiment, the invention comprises the step of providing the source of protons, such as an acid, at a concentration and/or amount capable of kinetically driving catalysis formation, activation, hydrogenation and/or regeneration steps. In an embodiment, the invention comprises the step of providing the source of electrons at a concentration and/or amount capable of kinetically driving catalysis formation and/or regeneration. In an embodiment, the invention comprises the step of replenishing the concentrations and/or amounts of the transition metal catalyst, source of protons, source of electrons, molecular nitrogen or combinations thereof.

In an embodiment, the transition metal catalyst is a metal complex further comprises a N 2 group, for example, wherein the transition metal catalyst is characterized by a metal atom providing for binding and subsequent functionalization of a N 2 substrate. In an embodiment, for example, the transition metal catalyst has the formula (FX3): (L)Z(N 2 ) − (FX3), wherein L is the phosphine ligand, and Z is the transition metal atom, and optionally wherein the N 2 is bound to the transition metal atom. In embodiments, the process further comprises the step of protonating the transition metal catalyst so as to generate a hydrogenated metal-N 2 complex, such as a hydrogenated metal-N 2 complex characterized by one or more N—H bonds. In an embodiment, for example, protonation occurs via contacting the transition metal catalyst with an acid, such as HBAr F 4 (hydro tetrakis[(3,5-trifluoromethyl)phenyl]borate), HOTf (triflic acid), HX, HBF 4 , H(Al(OR) 4 ) where R can be fluorinated, ArNH 3 +X or a combination of these, wherein X is a halogen (F, Cl, Br, I). In an embodiment, for example, the hydrogenated metal-N 2 complex has the formula (FX4): (L)Z(N x H y ) (FX4); wherein x is 1 or 2; y is 1, 2, 3, 4 or 5; L is the phosphine ligand and Z is the transition metal atom. In some embodiments, the hydrogenated metal-N 2 complex, for example having formula (FX4), is an ion such as an ion having a charge equal to +2, +1, −1 or −2. In an embodiment, for example, the hydrogenated metal-N 2 complex has the formula (FX5A) or (FX5B): (L)Z(NH 2 ) (FX5A) or (L)Z(NH 3 ) + (FX5B); wherein the phosphine ligand and Z is the transition metal atom. In an embodiment, the catalytic process further comprises reductive protonation of the hydrogenated metal-N 2 complex, for example using Na, KC 8 , Na/Hg, NaBH 4 − , Mg, Zn or any combination of these, thereby generating the reduction product and regenerating the transition metal catalyst.

The invention is further inclusive of transition metal catalyst precursors for generating the present transition metal catalyst. In an embodiment, for example, the invention provides methods wherein a transition metal catalyst precursors is contacted with molecular nitrogen to generate the transition metal catalyst. In an embodiment, for example, the catalytic process further comprises: (i) providing a transition metal precursor comprising a precursor transition metal complex comprising the transition metal atom and the phosphine ligand (L); and (ii) contacting the transition metal catalyst precursor with molecular nitrogen in the presence of an acid and reductant, thereby generating the transition metal catalyst comprising a N 2 adduct of the transition metal catalyst precursor. In an embodiment, the transition metal catalyst precursor has the formula (L)Z + (FX6), wherein L is the phosphine ligand and Z is the transition metal atom. In an embodiment, the transition metal catalyst is generated via reduction of the transition metal catalyst precursor, wherein the reducing agent is Na, Na/Hg or KC 8 .

The transition metal atom, as well as the axial donor atom within the ligand to which it is bonded, may play an important role in certain catalysts and catalytic methods of the invention. In an embodiment, for example, an N 2 is directly bonded to the transition metal atom, thereby forming an N 2 adduct. In an embodiment, the transition metal catalyst further comprises one or more counter ions. A wide range of counter ions are useful in the present compositions and methods, for example, Na + or K + , optionally solvated or coordinated by a crown, cryptand, or any donor group in principle.

In an embodiment, for example, the transition metal catalyst is a mononuclear metal complex. In an embodiment, for example, the transition metal catalyst is a metal coordination complex characterized by a coordination number of 3, 4, 5 or 6, and in some embodiments a coordination number of 4 or 5. In an embodiment, the transition metal atom of the transition metal catalyst is Fe. In an embodiment, the transition metal atom of the transition metal catalyst is Fe and characterized by an oxidation state of Fe(−1), Fe(0), Fe(I), Fe(II), Fe(III), or Fe(IV). For example, an LFe(N) precatalyst can have an oxidation state of Fe(IV), and can be an intermediate of catalysis. In an embodiment, the transition metal atom of the transition metal catalyst is Co.

In an embodiment, the phosphine ligand is a polydentate ligand, such as a tridentate or tetradentate ligand. In an embodiment, for example, the phosphine ligand is a tripodal trisphospine ligand having a boron, carbon, silicon and/or or phosphorous axial donor atom(s). In an embodiment, the phosphine ligand has an aryl backbone comprising at least one of L 1 , L 2 and L 3 independently comprising C 5 -C 10 arylene or C 5 -C 10 heteroarylene. In an embodiment, for example, the phosphine ligand comprises one or more cyclohexylamine ring systems.

The invention includes transition metal catalysts comprising a metal complex, wherein at least one of, and optionally all of, L 1 , L 2 , L 3 , R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 and R 8 are substituted with one or more heteroatoms or heavy atom analogues. In an embodiment, for example, a transition metal catalysts comprises a metal complex, wherein at least one of, and optionally all of, L 1 , L 2 and L 3 is independently substituted with one or more heteroatoms, such as one or more heteroatoms selected from the group consisting of O, N, or Si. In an embodiment, for example, a transition metal catalyst comprises a metal complex, wherein at least one of, and optionally all of, R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 and R 8 is independently substituted with one or more heteroatoms, such as one or more heteroatoms selected from the group consisting of O, N, or Si, or heavy atom analogues, such as As.

In an embodiment, for example, the ligand of the transition metal catalyst has the formula (FX7A), (FX7B) or (FX7C):

wherein X 1 , R 1 , R 2 , R 3 , R 4 , R 5 , R 6 and R 7 , are as set forth in formulas (FX1A), (FX1B) and (FX1C). In an embodiment, the ligand of the metal complex is neutral or is charged, such as having a charge of −1 or −2. In an embodiment, for example, the metal complex further comprises N 2 and has the formula (FX8A), (FX8B) or (FX8C):

wherein Z is the transition metal atom and wherein X 1 , R 1 , R 2 , R 3 , R 4 , R 5 , R 6 and R 7 , are as set forth in formulas (FX1A), (FX1B) and (FX1C).

In an embodiment, for example, the ligand of the transition metal catalyst has the formula (FX9A), (FX9B) or (FX9C):

wherein R 1 , R 2 , R 3 , R 4 , R 5 , and R 6 are as set forth in formulas (FX1A), (FX1B) and (FX1C). In an embodiment, for example, each of R 1 , R 2 , R 3 , R 4 , R 5 , R 6 and R 7 is independently isopropyl, phenyl or cyclohexyl. In an embodiment, for example, one of R 1 and R 2 , R 3 and R 4 , and R 5 and R 6 is different from the others. In an embodiment, the ligand of the metal complex is neutral or is charged, such as having a charge of −1 or −2.

In an embodiment, for example, the ligand of the transition metal catalyst has formula (FX10A), (FX10B) or (FX10C):

wherein iPr is isopropyl, Ph is phenyl, and Cy is cyclohexyl. In an embodiment, for example, the ligand of the transition metal catalyst has the formula (FX10A), (FX10B), (FX10C) or (FX10D):

wherein iPr is isopropyl, Ph is phenyl and Cy is cyclohexyl. In an embodiment, the ligand of the metal complex is neutral or is charged, such as having a charge of −1 or −2.

In an embodiment, the transition metal catalyst has the formula [(TP R B)Fe(N 2 )] − , [(CP R 3 )Fe(N 2 )] − , [(SiP R 3 )Fe(N 2 )] − , [(TP R B)Co(N 2 )] − , [(CP R 3 )Co(N 2 )] − , or [(SiP R 3 )Co(N 2 )] − , wherein TP R B is a tris(phosphinoaryl)borane ligand, CP R 3 is a tris(phosphinoaryl)alkyl ligand and SiP R 3 is tris(phosphinoaryl)silyl ligand.

An important feature of the invention is the capability of the catalysts and catalytic systems to reduce molecular nitrogen at ambient temperatures or lower and atmospheric pressures or lower using a versatile range of reaction conditions. In an embodiment, for example, the transition metal catalyst and the molecular nitrogen are each dissolved in a solvent; wherein the step of contacting the transition metal catalyst with the source of protons and the source of electrons occurs within a solution. Criteria for selection of an appropriate solvent include stability to the reductant and the acid used. In an embodiment, for example, the solution is a nonaqueous solution. In an embodiment, the solvent is selected from the group consisting of: diethyl ether, related ethereal solvent (such as tetrahydrofuran or dimethoxyethane or diglym), or an aromatic or alkane solvent such as methylcyclohexane, or mixtures thereof. In some embodiments, for example, the solvent has heteroatoms other than oxygen (including those with N or S).

In an embodiment, for example, at least one of the molecular nitrogen the transition metal catalyst, the source of protons and the source of electrons, and optionally all of, are provided in a solution comprising one or more solvents. In an embodiment, the one or more solvents is one or more nonaqueous solvents. In an embodiment, for example, the transition metal catalyst is a homogeneous catalyst, wherein the transition metal catalyst, the source of protons, the source of electrons and the molecular nitrogen are provided in contact in the solution. In an embodiment, for example, the transition metal catalyst is a heterogeneous catalyst, where the transition metal, the source of protons and the source of electrons and the molecular nitrogen are provided in the solution and provided in contact with the transition metal catalyst provided in the solid phase. In an embodiment, for example, the transition metal catalyst is a heterogeneous catalyst, where the source of protons and the source of electrons and the molecular nitrogen are provided in the solution, the molecular nitrogen is provided in the gas phase, which are provided in contact with the transition metal catalyst provided in the solid phase.

In an embodiment, for example, the concentration of the transition metal catalyst in the solution is selected from the range of 0.01 mM to 10 mM. In an embodiment, for example, the concentration of the molecular nitrogen in the solution is selected from the range of 1×10 −4 M to 1 M.

A wide variety of proton sources is useful in the present invention, including a range of acids. In an embodiment, the source of protons is one or more acids, for example, one or more acids selected from the group consisting of: HBAr F 4 (hydro tetrakis[(3,5-trifluoromethyl)phenyl]borate), HOTf (triflic acid), HX, HBF 4 , H(Al(OR) 4 ) where R can be fluorinated, ArNH 3 +X or a combination of these; wherein X is a halogen (e.g., F, Cl, Br, or I). In an embodiment, the concentration of the one or more acids is selected from the range of 0.01-5 M.

A wide variety of electron sources is useful in the present invention, including a range of reductants. In embodiments, the source of electrons is one or more reductants, for example, one or more reductants selected from the group consisting of Na, KC 8 , Na/Hg, NaBH 4 − , Mg, Zn or any combination of these. In an embodiment, the concentration of the one or more reductants is selected from the range of 0.1-100 M.

In an embodiment, the transition metal catalyst is provided as a homogeneous catalyst, for example, wherein the catalyst and molecular nitrogen are provided, and provided in contact with each other, in the same phase, such as in solution. Alternative, the present transition metal catalysts may be provided as heterogeneous catalyst, for example, wherein the catalyst is provided as a phase different from the molecular nitrogen, such as wherein the transition metal catalyst is provided as a solid and contacted by molecular nitrogen in gas or solution phase. Examples of solvents useful for homogeneous or heterogeneous catalyst of the invention include nonaqueous solvents such as diethyl ether, diethyl ether, tetrahydrofuran, dimethoxyethane, diglym, methylcyclohexane or any combination of these.

The catalysts and catalytic processes disclosed herein are efficient and provide benefits over conventional functional biomimetic catalyst systems for the reduction of molecular nitrogen. In an embodiment, the catalytic process generates at least 1, and optionally 2 or 10, reduction product equivalents per transition metal catalyst equivalent.

In an aspect, the invention provides a catalyst formulation for reduction of molecular nitrogen (N 2 ) to generate a reduction product, the formulation comprising: (i) a transition metal catalyst comprising a metal complex comprising a transition metal atom selected from the group consisting of Fe and Co, and a phosphine ligand (L); (ii) a source of protons comprising one or more acids; and (iii) a source of electrons comprising one or more reductants. In an embodiment, the metal complex of the transition metal catalyst further comprises a N 2 group, for example, an N 2 group bound to the metal atom. In an embodiment, for example, the catalyst formulation is provided in solution, for example, further comprising a nonaqueous solvent. As will be generally understood by those having skill in the art, any of the transition metal catalysts, metal complexes, transition metal catalyst precursors, sources of electrodes, sources of protons and solvents described herein can be used in the present catalyst formulations. In an embodiment, for example the acid is HBAr F 4 (hydro tetrakis[(3,5-trifluoromethyl)phenyl]borate). In an embodiment, for example, the one or more reductants are selected from the group consisting of Na, KC 8 , Na/Hg, NaBH 4 − , Mg, Zn or any combination of these. In an embodiment, the catalyst formulation of the invention is in a substantially purified state.

In another aspect, the present invention provides metal complex compositions, for example, for applications in chemical synthesis. In an embodiment, the invention provides a metal complex comprising a transition metal atom selected from the group consisting of Fe and Co, and a phosphine ligand (L); wherein the ligand (L) has the formula (FX1A), (FX1B) or (FX1C):

wherein X 1 is B, C, Si or P; each of L 1 , L 2 and L 3 is independently a substituted or unsubstituted C 1 -C 10 alkylene, C 3 -C 10 cycloalkylene, C 5 -C 10 arylene, or C 5 -C 10 heteroarylene; each of R 1 , R 2 , R 3 , R 4 , R 5 , R 6 and R 7 is independently hydrogen or a substituted or unsubstituted C 1 -C 8 alkyl, C 3 -C 8 cycloalkyl, C 5 -C 8 aryl, C 5 -C 8 heteroaryl, C 1 -C 18 acyl, C 2 -C 8 alkenyl, C 2 -C 8 alkynyl, or —P(OR 8 ) 2 , wherein each R 8 is independently H, C 1 -C 8 alkyl, C 3 -C 8 cycloalkyl, C 5 -C 8 aryl or C 5 -C 8 heteroaryl with the proviso that when the ligand (L) is tris(o-phosphinoaryl)boran, then the metal atom is Co. In an embodiment, the metal complex further comprises a N 2 group, for example, an N 2 group bound to the metal atom. In an embodiment, for example, the metal complex has the formula: (L)Z(N 2 ) − (FX3). In an embodiment, for example, the invention provides a metal complex having formula (FX1A), (FX1B) or (FX1C), wherein the metal atom is Fe. In an embodiment, for example, the invention provides a metal complex having formula (FX1A), (FX1B) or (FX1C), wherein the metal atom is Co. In an embodiment, for example, the invention provides a metal complex having formula (FX1A), (FX1B) or (FX1C), wherein X 1 is C or Si. In an embodiment, for example, the invention provides a metal complex having formula (FX1A), (FX1B) or (FX1C), wherein at least one of, and optionally all of, R 1 , R 2 , R 3 , R 4 , R 5 , R 6 and R 7 is not isopropyl. In an embodiment, for example, the invention provides a metal complex having formula (FX1A), (FX1B) or (FX1C), wherein at least one of, and optionally all of, R 1 , R 2 , R 3 , R 4 , R 5 , R 6 and R 7 is phenyl and cyclohexyl. In an embodiment, the metal complex of the invention is in a substantially purified state.

Without wishing to be bound by any particular theory, there may be discussion herein of beliefs or understandings of underlying principles relating to the devices and methods disclosed herein. It is recognized that regardless of the ultimate correctness of any mechanistic explanation or hypothesis, an embodiment of the invention can nonetheless be operative and useful.

FIGS. 1A and 1B . Schematics of catalytic cycle. Schematic depictions of catalytic cycles involving transition metal catalysts of the invention.

FIG. 2 . Chemical line representations of the FeMo-cofactor of nitrogenase. A schematic depiction of N 2 binding and reduction at an Fe site via limiting alternating (top) and distal (bottom) mechanisms. The drawing emphasizes a possible hemi-labile role for the interstitial C-atom with respect to an Fe—N 2 binding site.

FIG. 3 . Stoichiometric (TPB)Fe—N 2 model reactions. (a) N 2 binding to [(TPB)Fe][BAr F 4 ] under electron-loading to generate [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ]. (b) Reductive protonolysis of (TPB)Fe(NH 2 ) to release NH 3 with concomitant N 2 uptake. (c) Generation of [(TPB)Fe(NH 3 )][BAr F 4 ] and other (TPB)Fe-species upon addition of acid to [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ] at low temperature, followed by warming and then addition of base. 12-C-4 is an abbreviation for 12-crown-4. Note: TPB=tris(phosphine)borane.

FIG. 4 . Spectral data for ammonia analysis, and catalyst poisoning. (a) 1 H NMR spectrum (300 MHz) of [ 14 NH 4 ][Cl] in DMSO-d 6 produced from a catalytic run under 14 N 2 (top) and of [ 15 NH 4 ][Cl] in DMSO-d 6 produced from an independent catalytic run under 1 atm 15 N 2 . (b) Representative optical data for NH 3 product analysis using the indophenol method from catalytic runs using the standard conditions with the precursors indicated. (c) H 2 binds to (TPB)Fe(N 2 ) to generate (TPB)(μ-H)Fe(N 2 )(H), which itself is ineffective for the catalytic generation of NH 3 from N 2 under the standard conditions. Note: TPB=tris(phosphine)borane; DMSO=dimethylsulfoxide.

FIG. 5 . 1 H NMR spectrum of the reaction mixture following protonation of [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ] compared with an authentic sample of [(TPB)Fe(NH 3 )][BAr F 4 ].

FIG. 6 . Stacked EPR spectra at 77 K of [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ], the yellow species generated upon addition of acid to [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ], and the green-yellow species generated from (TPB)Fe≡N(p-C 6 H 4 OMe) and [Fc][BAr F 4 ] at 77 K.

FIG. 7 . 1 H NMR spectrum of [ 14 NH 4 ][Cl] produced from [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ], HBAr F 4 . 2 Et 2 O, and KC 8 under 14 N 2 .

FIG. 8 . 1 H NMR spectrum of [ 15 NH 4 ][Cl] produced from [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ], HBAr F 4 .2 Et 2 O, and KC 8 under 15 N 2 .

FIG. 9 . Calibration curves for NH 3 and N 2 H 4 UV-Vis quantification.

FIG. 10 . IR spectra of addition of 10 equiv HBAr F 4 .2 Et 2 O to [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ], followed by 12 equiv KC 8 .

FIG. 11 . 31 P{ 1 H} NMR spectra of addition of 10 equiv HBAr F 4 .2 Et 2 O to [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ], followed by 12 equiv KC 8 .

FIG. 12 . (Top) Structure of the FeMo cofactor of nitrogenase, showing a putative site for dinitrogen binding and highlighting the trigonal bipyramidal coordination environment at Fe. Possible sites of H-atoms on cofactor prior to N 2 binding not shown. (Bottom) Possible role of Lewis acidic (LA) or aryl substituents in stabilizing ionic character in the N 2 —Fe—C alkyl interaction.

FIG. 13 . Select trigonal bipyramidal scaffolds and the (CP iPr 3 )FeN 2 − system.

FIG. 14 . Crystal structures of {(CP iPr 3 )H}FeI 2 (6, top left), {(CP iPr 3 )H}FeBr (8, top right), and (CP iPr 3 )Fe(H)(N 2 ) (9, bottom). Ellipsoids shown at 50% probability; hydrogen atoms (except the triarylmethine C—H and Fe—H hydride) and solvent molecules omitted for clarity.

FIG. 15 . Crystal structures of (CP iPr 3 )FeN 2 (11, left), (CP iPr 3 )FeN 2 − (12[K(Et 2 O) 3 ], center, ethyl groups of coordinated Et 2 O molecules omitted), and (CP iPr 3 )FeN 2 + (13, right). Ellipsoids are shown at 50% probability and hydrogen atoms are omitted for clarity.

FIG. 16 . Cyclic voltammogram of 11; scan rate 0.5 V/s.

FIG. 17 . (A) Isocontour plot of the Fe—C alkyl σ bond of 12[K(Et 2 O) 3 ] located from NBO analyses. (B) Contour plot of one of the C aryl π* orbitals which accepts delocalized electron density from the Fe—C alkyl σ bond.

FIG. 18 . Spectroscopic analyses of reaction mixtures following the catalytic production of NH 3 using 12[K(Et 2 O) 0.5 ] as a catalyst. Symbols indicate characteristic resonances attributed to 9, 10, and 11. (A),(C) 1 H NMR and IR spectra of a post-catalytic reaction mixture using 10 equiv. of [H(Et 2 O) 2 ][BAr F 4 ] and 12 equiv. of KC 8 . (B),(D) 1 H NMR and IR spectra of a post-catalytic reaction mixture using 38 equiv. of [H(Et 2 O) 2 ][BAr F 4 ] and 40 equiv. of KC 8 .

FIG. 19 . 1 H NMR of 2 (DMSO-d 6 , 300 MHz).

FIG. 20 . 13 C NMR of 2 (DMSO-d 6 , 75 MHz).

FIG. 21 . 1 H NMR of 3 (CDCl 3 , 300 MHz).

FIG. 22 . 13 C NMR of 3 (CDCl 3 , 75 MHz).

FIG. 23 . 1 H NMR of 4 (aromatic region) in DMSO-d 6 (300 MHz).

FIG. 24 . 1 H NMR of 5 (CDCl 3 , 300 MHz).

FIG. 25 . 13 C NMR of 5 (CDCl 3 , 75 MHz).

FIG. 26 . 1 H NMR of 1 (C 6 D 6 , 300 MHz) with inset showing coupling to central methine proton.

FIG. 27 . 13 C NMR of 1 (C 6 D 6 , 75 MHz).

FIG. 28 . 31 P NMR of 1 (C 6 D 6 , 300 MHz).

FIG. 29 . 1 H NMR of 6 (C 6 D 6 ) with inset showing peak at 180 ppm. No other peaks appear outside the range depicted.

FIG. 30 . 1 H NMR of 7 (C 6 D 6 , 300 MHz), unpurified, generated by reduction of 6 with sodium amalgam in benzene.

FIG. 31 . 1 H NMR of 9 (C 6 D 6 , 300 MHz) with inset showing 31 P NMR.

FIG. 32 . IR (thin film deposited from benzene) of 9; v(NN)=2046 cm −1 ; v(FeH)=1920 cm −1 .

FIG. 33 . 1 H NMR of 8 (C 6 D 6 , 300 MHz), unpurified, generated by reduction of CP3HFeBr2 with isopropyl magnesium chloride in toluene. For unknown reasons the reaction appears to yield a mixture of two closely related paramagnetic species.

FIG. 34 . 1 H NMR of 10 (C 6 D 6 , 300 MHz).

FIG. 35 . 1 H NMR of 11 (C 6 D 6 , 300 MHz).

FIG. 36 . IR (thin film deposited from benzene) of 11. v(NN)=1992 cm −1 .

FIG. 37 . 1 H NMR of 12 (d 8 -THF, 300 MHz).

FIG. 38 . 31 P NMR of 12.

FIG. 39 . IR spectrum of 12 as a thin film deposited from dimethoxyethane.

FIG. 40 . 1 H NMR of 12[K(12-crown-4) 2 ] in d 8 -THF.

FIG. 41 . IR spectrum of 12[K(12-crown-4) 2 ] (thin film from THF).

FIG. 42 . 1 H NMR (4:1 C 6 D 6 /THF-d 8 under N 2 , 300 MHz, 298 K) of 13.

FIG. 43 . IR of 13 as a thin film deposited from THF.

FIG. 44 . UV-Vis spectra of 13 under N 2 and under static vacuum (after three freeze-pump-thaw cycles). 13 is in a solution of 3:1 Et 2 O:THF at a concentration of 0.54 mM.

FIG. 45 . 1 H NMR of 14, C 6 D 6 .

FIG. 46 . 1 H NMR of 14, showing paramagnetic regions magnified to show minor contamination with CP iPr 3 FeN 2 (11).

FIG. 47 . 31 P NMR of 14 (C 6 D 6 ).

FIG. 48 . IR (thin-film from C 6 D 6 ) of 14. Peak at 1992 cm −1 is contamination by CP iPr 3 FeN 2 .

FIG. 49 . NMR of reaction mixture after catalysis with internal standard (1,3,5-trimethoxybenzene).

FIG. 50 . Calibration curve for NH 3 quantification via indophenol method.

FIG. 51 . Calibration curve for UV-Vis quantification of hydrazine.

FIG. 52 . X-Ray Diffraction (XRD) structures of complexes 1 (A) and 2 (B) with hydrogen atoms and counterion (for B) omitted for clarity. See Table 24 for selected bond lengths and angles.

FIG. 53 . XRD structures of the cores of complexes 3 (A), 4 (B), and 5 (C). See Table 24 for selected distances and angles.

FIG. 54 . X-Band EPR spectra for complexes 1-6. Conditions for 1: Toluene, 8 K; 2: 2:1 Toluene:Et 2 O, 10 K 3: 2-MeTHF, 10 K; 4: 2-MeTHF, 10 K; 5: 2-MeTHF, 10 K; 6: Toluene, 10 K.

FIG. 55 . 1 H NMR Spectrum of (TPB)FeMe (1).

FIG. 56 . 1 H NMR Spectrum of [(TPB)Fe][BAr F 4 ] (2).

FIG. 57 . 1 H NMR Spectrum of [(TPB)Fe(N 2 H 4 )][BAr F 4 ] (3).

FIG. 58 . 1 H NMR Spectrum of [(TPB)Fe(NH 3 )][BAr F 4 ] (4).

FIG. 59 . 1 H NMR Spectrum of (TPB)FeNH 2 (5).

FIG. 60 . 1 H NMR Spectrum of (TPB)FeOH (6).

FIG. 61 . 10 K EPR Spectrum of (TPB)FeMe (1).

FIG. 62 . 10 K EPR Spectrum of [(TPB)Fe][BAr F 4 ] (2).

FIG. 63 . 10 K EPR Spectrum of [(TPB)Fe(N 2 H 4 )][BAr F 4 ] (3).

FIG. 64 . 10 K EPR Spectrum of [(TPB)Fe(NH 3 )][BAr F 4 ] (4).

FIG. 65 . 10 K EPR Spectrum of (TPB)FeNH 2 (5).

FIG. 66 . 10 K EPR Spectrum of (TPB)FeOH (6).

FIG. 67 . UV-Vis Spectrum of (TPB)FeMe (1).

FIG. 68 . UV-Vis Spectrum of [(TPB)Fe][BAr F 4 ] (2).

FIG. 69 . UV-Vis Spectrum of [(TPB)Fe(N 2 H 4 )][BAr F 4 ] (3).

FIG. 70 . UV-Vis Spectrum of [(TPB)Fe(NH 3 )][BAr F 4 ] (4).

FIG. 71 . UV-Vis Spectrum of (TPB)FeNH 2 (5).

FIG. 72 . UV-Vis Spectrum of (TPB)FeOH (6).

FIG. 73 . Titration of THF into an ethereal solution of 2.

FIG. 74 . NMR traces of the monitored decomposition of 3 to 4.

FIG. 75 . Kinetic plots of the monitored decomposition of 3 to 4.

FIG. 76 . Geometries of [(TPB)Fe] + and [(Me 2 PhP) 3 Fe] + optimized at the B3LYP/6-31 G(d) level.

FIG. 77 . MO and spin density diagram of [(TPB)Fe] + optimized at the B3LYP/6-31 G(d) level.

FIG. 78 . MO diagram offering a tentative explanation for the T-shaped distortion of 2.

FIG. 79 . Variable Temperature Magnetic data for 2-5.

FIG. 80 . Crystal Structure for (TPB)FeOH (6).

FIG. 81 . Complexes synthesized and studied towards N 2 reduction in this work.

FIG. 82 . XRD structures of complexes 1 (E), 3 (D), 5 (A), 8 (B), 9 (C), and 10 (F) with ellipsoids at 50% and hydrogens and counterions omitted for clarity. Fe atoms are shown in orange, P in purple, B in tan, Cl in green, and N in blue. See Table 32 for bonding metrics.

FIG. 83 . Catalytic competence and vibrational spectroscopy data for select (P 3 X)M(N 2 ) − complexes. Data for M=Fe, X=B is from refs. 8 and 11; data for M=Fe, X=C is from ref. 9; data for M=Fe, X=Si is from ref.s 9 and 13; and data for M=Co, X=B is from this work. a IR of a KBr pellet b IR of a thin film from evaporation of solvent.

FIG. 84 . Calibration curve for NH 3 quantification by indophenol method.

FIG. 85 . Calibration curve for UV-vis quantification of hydrazine.

FIG. 86 . Cyclic voltammagram of (TPB)Co(N 2 ) (1) scanning oxidatively (left) and reductively (right) at 100 mV/sec in THF with 0.1 M TBAPF 6 electrolyte.

FIG. 87 . Temperature dependence of the magnetic susceptibility of [(TPB)Co][BAr F 4 ] (3) as measured by SQUID magnetometry.

In an embodiment, a composition or compound of the invention, such as a metal catalyst composition or formulation, is isolated or substantially purified. In an embodiment, an isolated or purified compound is at least partially isolated or substantially purified as would be understood in the art. In an embodiment, a substantially purified composition, compound or formulation of the invention has a chemical purity of 95%, optionally for some applications 99%, optionally for some applications 99.9%, optionally for some applications 99.99%, and optionally for some applications 99.999% pure.

Many of the molecules disclosed herein contain one or more ionizable groups. Ionizable groups include groups from which a proton can be removed (e.g., —COOH) or added (e.g., amines) and groups that can be quaternized (e.g., amines). All possible ionic forms of such molecules and salts thereof are intended to be included individually in the disclosure herein. With regard to salts of the compounds herein, one of ordinary skill in the art can select from among a wide variety of available counterions that are appropriate for preparation of salts of this invention for a given application. In specific applications, the selection of a given anion or cation for preparation of a salt can result in increased or decreased solubility of that salt.

The compounds of this invention can contain one or more chiral centers. Accordingly, this invention is intended to include racemic mixtures, diasteromers, enantiomers, tautomers and mixtures enriched in one or more stereoisomer. The scope of the invention as described and claimed encompasses the racemic forms of the compounds as well as the individual enantiomers and non-racemic mixtures thereof.

As used herein, the term “group” may refer to a functional group of a chemical compound. Groups of the present compounds refer to an atom or a collection of atoms that are a part of the compound. Groups of the present invention may be attached to other atoms of the compound via one or more covalent bonds. Groups may also be characterized with respect to their valence state. The present invention includes groups characterized as monovalent, divalent, trivalent, etc. valence states.

As used herein, the term “substituted” refers to a compound wherein a hydrogen is replaced by another functional group.

As is customary and well known in the art, hydrogen atoms in formulas (FX1)-(FX10) are not always explicitly shown, for example, hydrogen atoms bonded to the carbon atoms of alkylene groups and/or alicyclic rings are not always explicitly shown in formulas (FX1)-(FX10). The structures provided herein, for example in the context of the description of formulas (FX1)-(FX10), are intended to convey to one of reasonable skill in the art the chemical composition of compounds of the methods and compositions of the invention, and as will be understood by one of skill in the art, the structures provided do not indicate the specific positions of atoms and bond angles between atoms of these compounds.

Alkyl groups include straight-chain, branched and cyclic alkyl groups. Alkyl groups include those having from 1 to 30 carbon atoms. Alkyl groups include small alkyl groups having 1 to 3 carbon atoms. Alkyl groups include medium length alkyl groups having from 4-10 carbon atoms. Alkyl groups include long alkyl groups having more than 10 carbon atoms, particularly those having 10-30 carbon atoms. The term cycloalkyl specifically refers to an alky group having a ring structure such as ring structure comprising 3-30 carbon atoms, optionally 3-20 carbon atoms and optionally 2-10 carbon atoms, including an alkyl group having one or more rings. Cycloalkyl groups include those having a 3-, 4-, 5-, 6-, 7-, 8-, 9- or 10-member carbon ring(s) and particularly those having a 3-, 4-, 5-, 6-, 7- or 8-member ring(s). The carbon rings in cycloalkyl groups can also carry alkyl groups. Cycloalkyl groups can include bicyclic and tricycloalkyl groups. Alkyl groups are optionally substituted. Substituted alkyl groups include among others those which are substituted with aryl groups, which in turn can be optionally substituted. Specific alkyl groups include methyl, ethyl, n-propyl, iso-propyl, cyclopropyl, n-butyl, s-butyl, t-butyl, cyclobutyl, n-pentyl, branched-pentyl, cyclopentyl, n-hexyl, branched hexyl, and cyclohexyl groups, all of which are optionally substituted. Substituted alkyl groups include fully halogenated or semihalogenated alkyl groups, such as alkyl groups having one or more hydrogens replaced with one or more fluorine atoms, chlorine atoms, bromine atoms and/or iodine atoms. Substituted alkyl groups include fully fluorinated or semifluorinated alkyl groups, such as alkyl groups having one or more hydrogens replaced with one or more fluorine atoms. Substituted alkyl groups may include substitution to incorporate one or more silyl groups, for example wherein one or more carbons are replaced by Si.

An alkoxy group is an alkyl group that has been modified by linkage to oxygen and can be represented by the formula R—O and can also be referred to as an alkyl ether group. Examples of alkoxy groups include, but are not limited to, methoxy, ethoxy, propoxy, butoxy and heptoxy. Alkoxy groups include substituted alkoxy groups wherein the alky portion of the groups is substituted as provided herein in connection with the description of alkyl groups. As used herein MeO— refers to CH 3 O—.

Alkenyl groups include straight-chain, branched and cyclic alkenyl groups. Alkenyl groups include those having 1, 2 or more double bonds and those in which two or more of the double bonds are conjugated double bonds. Alkenyl groups include those having from 2 to 20 carbon atoms. Alkenyl groups include small alkenyl groups having 2 to 3 carbon atoms. Alkenyl groups include medium length alkenyl groups having from 4-10 carbon atoms. Alkenyl groups include long alkenyl groups having more than 10 carbon atoms, particularly those having 10-20 carbon atoms. Cycloalkenyl groups include those in which a double bond is in the ring or in an alkenyl group attached to a ring. The term cycloalkenyl specifically refers to an alkenyl group having a ring structure, including an alkenyl group having a 3-, 4-, 5-, 6-, 7-, 8-, 9- or 10-member carbon ring(s) and particularly those having a 3-, 4-, 5-, 6-, 7- or 8-member ring(s). The carbon rings in cycloalkenyl groups can also carry alkyl groups. Cycloalkenyl groups can include bicyclic and tricyclic alkenyl groups. Alkenyl groups are optionally substituted. Substituted alkenyl groups include among others those that are substituted with alkyl or aryl groups, which groups in turn can be optionally substituted. Specific alkenyl groups include ethenyl, prop-1-enyl, prop-2-enyl, cycloprop-1-enyl, but-1-enyl, but-2-enyl, cyclobut-1-enyl, cyclobut-2-enyl, pent-1-enyl, pent-2-enyl, branched pentenyl, cyclopent-1-enyl, hex-1-enyl, branched hexenyl, cyclohexenyl, all of which are optionally substituted. Substituted alkenyl groups include fully halogenated or semihalogenated alkenyl groups, such as alkenyl groups having one or more hydrogens replaced with one or more fluorine atoms, chlorine atoms, bromine atoms and/or iodine atoms. Substituted alkenyl groups include fully fluorinated or semifluorinated alkenyl groups, such as alkenyl groups having one or more hydrogen atoms replaced with one or more fluorine atoms.

Aryl groups include groups having one or more 5-, 6-, 7- or 8-member aromatic rings, including heterocyclic aromatic rings. The term heteroaryl specifically refers to aryl groups having at least one 5-, 6-, 7- or 8-member heterocyclic aromatic rings. Aryl groups can contain one or more fused aromatic rings, including one or more fused heteroaromatic rings, and/or a combination of one or more aromatic rings and one or more nonaromatic rings that may be fused or linked via covalent bonds. Heterocyclic aromatic rings can include one or more N, O, or S atoms in the ring. Heterocyclic aromatic rings can include those with one, two or three N atoms, those with one or two O atoms, and those with one or two S atoms, or combinations of one or two or three N, O or S atoms. Aryl groups are optionally substituted. Substituted aryl groups include among others those that are substituted with alkyl or alkenyl groups, which groups in turn can be optionally substituted. Specific aryl groups include phenyl, biphenyl groups, pyrrolidinyl, imidazolidinyl, tetrahydrofuryl, tetrahydrothienyl, furyl, thienyl, pyridyl, quinolyl, isoquinolyl, pyridazinyl, pyrazinyl, indolyl, imidazolyl, oxazolyl, thiazolyl, pyrazolyl, pyridinyl, benzoxadiazolyl, benzothiadiazolyl, and naphthyl groups, all of which are optionally substituted. Substituted aryl groups include fully halogenated or semihalogenated aryl groups, such as aryl groups having one or more hydrogens replaced with one or more fluorine atoms, chlorine atoms, bromine atoms and/or iodine atoms. Substituted aryl groups include fully fluorinated or semifluorinated aryl groups, such as aryl groups having one or more hydrogens replaced with one or more fluorine atoms. Aryl groups include, but are not limited to, aromatic group-containing or heterocylic aromatic group-containing groups corresponding to any one of the following: benzene, naphthalene, naphthoquinone, diphenylmethane, fluorene, anthracene, anthraquinone, phenanthrene, tetracene, tetracenedione, pyridine, quinoline, isoquinoline, indoles, isoindole, pyrrole, imidazole, oxazole, thiazole, pyrazole, pyrazine, pyrimidine, purine, benzimidazole, furans, benzofuran, dibenzofuran, carbazole, acridine, acridone, phenanthridine, thiophene, benzothiophene, dibenzothiophene, xanthene, xanthone, flavone, coumarin, azulene or anthracycline. As used herein, a group corresponding to the groups listed above expressly includes an aromatic or heterocyclic aromatic group, including monovalent, divalent and polyvalent groups, of the aromatic and heterocyclic aromatic groups listed herein provided in a covalently bonded configuration in the compounds of the invention at any suitable point of attachment. In embodiments, aryl groups contain between 5 and 30 carbon atoms. In embodiments, aryl groups contain one aromatic or heteroaromatic six-member ring and one or more additional five- or six-member aromatic or heteroaromatic ring. In embodiments, aryl groups contain between five and eighteen carbon atoms in the rings. Aryl groups optionally have one or more aromatic rings or heterocyclic aromatic rings having one or more electron donating groups, electron withdrawing groups and/or targeting ligands provided as substituents.

Arylalkyl groups are alkyl groups substituted with one or more aryl groups wherein the alkyl groups optionally carry additional substituents and the aryl groups are optionally substituted. Specific alkylaryl groups are phenyl-substituted alkyl groups, e.g., phenylmethyl groups. Alkylaryl groups are alternatively described as aryl groups substituted with one or more alkyl groups wherein the alkyl groups optionally carry additional substituents and the aryl groups are optionally substituted. Specific alkylaryl groups are alkyl-substituted phenyl groups such as methylphenyl. Substituted arylalkyl groups include fully halogenated or semihalogenated arylalkyl groups, such as arylalkyl groups having one or more alkyl and/or aryl groups having one or more hydrogens replaced with one or more fluorine atoms, chlorine atoms, bromine atoms and/or iodine atoms.

As used herein, the terms “alkylene” and “alkylene group” are used synonymously and refer to a divalent group derived from an alkyl group as defined herein. The invention includes compounds having one or more alkylene groups. Alkylene groups in some compounds function as attaching and/or spacer groups. Compounds of the invention may have substituted and/or unsubstituted C 1 -C 20 alkylene, C 1 -C 10 alkylene and C 1 -C 5 alkylene groups.

As used herein, the terms “cycloalkylene” and “cycloalkylene group” are used synonymously and refer to a divalent group derived from a cycloalkyl group as defined herein. The invention includes compounds having one or more cycloalkylene groups. Cycloalkyl groups in some compounds function as attaching and/or spacer groups. Compounds of the invention may have substituted and/or unsubstituted C 3 -C 20 cycloalkylene, C 3 -C 10 cycloalkylene and C 3 -C 5 cycloalkylene groups.

As used herein, the terms “arylene” and “arylene group” are used synonymously and refer to a divalent group derived from an aryl group as defined herein. The invention includes compounds having one or more arylene groups. In an embodiment, an arylene is a divalent group derived from an aryl group by removal of hydrogen atoms from two intra-ring carbon atoms of an aromatic ring of the aryl group. Arylene groups in some compounds function as attaching and/or spacer groups. Arylene groups in some compounds function as chromophore, fluorophore, aromatic antenna, dye and/or imaging groups. Compounds of the invention include substituted and/or unsubstituted C 3 -C 30 arylene, C 3 -C 20 arylene, C 3 -C 10 arylene and C 1 -C 5 arylene groups.

As used herein, the terms “heteroarylene” and “heteroarylene group” are used synonymously and refer to a divalent group derived from a heteroaryl group as defined herein. The invention includes compounds having one or more heteroarylene groups. In an embodiment, a heteroarylene is a divalent group derived from a heteroaryl group by removal of hydrogen atoms from two intra-ring carbon atoms or intra-ring nitrogen atoms of a heteroaromatic or aromatic ring of the heteroaryl group. Heteroarylene groups in some compounds function as attaching and/or spacer groups. Heteroarylene groups in some compounds function as chromophore, aromatic antenna, fluorophore, dye and/or imaging groups. Compounds of the invention include substituted and/or unsubstituted C 3 -C 30 heteroarylene, C 3 -C 20 heteroarylene, C 1 -C 10 heteroarylene and C 3 -C 5 heteroarylene groups.

As used herein, the terms “alkenylene” and “alkenylene group” are used synonymously and refer to a divalent group derived from an alkenyl group as defined herein. The invention includes compounds having one or more alkenylene groups. Alkenylene groups in some compounds function as attaching and/or spacer groups. Compounds of the invention include substituted and/or unsubstituted C 2 -C 20 alkenylene, C 2 -C 10 alkenylene and C 2 -C 5 alkenylene groups.

As used herein, the terms “cylcoalkenylene” and “cylcoalkenylene group” are used synonymously and refer to a divalent group derived from a cylcoalkenyl group as defined herein. The invention includes compounds having one or more cylcoalkenylene groups. Cycloalkenylene groups in some compounds function as attaching and/or spacer groups. Compounds of the invention include substituted and/or unsubstituted C 3 -C 20 cylcoalkenylene, C 3 -C 10 cylcoalkenylene and C 3 -C 5 cylcoalkenylene groups.

As used herein, the terms “alkynylene” and “alkynylene group” are used synonymously and refer to a divalent group derived from an alkynyl group as defined herein. The invention includes compounds having one or more alkynylene groups. Alkynylene groups in some compounds function as attaching and/or spacer groups. Compounds of the invention include substituted and/or unsubstituted C 2 -C 20 alkynylene, C 2 -C 10 alkynylene and C 2 -C 5 alkynylene groups.

As used herein, the term “halo” refers to a halogen group such as a fluoro (—F), chloro (—Cl), bromo (—Br), iodo (—I) or astato (—At).

The term “heterocyclic” refers to ring structures containing at least one other kind of atom, in addition to carbon, in the ring. Examples of such heteroatoms include nitrogen, oxygen and sulfur. Heterocyclic rings include heterocyclic alicyclic rings and heterocyclic aromatic rings. Examples of heterocyclic rings include, but are not limited to, pyrrolidinyl, piperidyl, imidazolidinyl, tetrahydrofuryl, tetrahydrothienyl, furyl, thienyl, pyridyl, quinolyl, isoquinolyl, pyridazinyl, pyrazinyl, indolyl, imidazolyl, oxazolyl, thiazolyl, pyrazolyl, pyridinyl, benzoxadiazolyl, benzothiadiazolyl, triazolyl and tetrazolyl groups. Atoms of heterocyclic rings can be bonded to a wide range of other atoms and functional groups, for example, provided as substituents.

The term “carbocyclic” refers to ring structures containing only carbon atoms in the ring. Carbon atoms of carbocyclic rings can be bonded to a wide range of other atoms and functional groups, for example, provided as substituents.

The term “alicyclic ring” refers to a ring, or plurality of fused rings, that is not an aromatic ring. Alicyclic rings include both carbocyclic and heterocyclic rings.

The term “aromatic ring” refers to a ring, or a plurality of fused rings, that includes at least one aromatic ring group. The term aromatic ring includes aromatic rings comprising carbon, hydrogen and heteroatoms. Aromatic ring includes carbocyclic and heterocyclic aromatic rings. Aromatic rings are components of aryl groups.

The term “fused ring” or “fused ring structure” refers to a plurality of alicyclic and/or aromatic rings provided in a fused ring configuration, such as fused rings that share at least two intra ring carbon atoms and/or heteroatoms.

As used herein, the term “alkoxyalkyl” refers to a substituent of the formula alkyl-O-alkyl.

As used herein, the term “polyhydroxyalkyl” refers to a substituent having from 2 to 12 carbon atoms and from 2 to 5 hydroxyl groups, such as the 2,3-dihydroxypropyl, 2,3,4-tri hydroxybutyl or 2,3,4,5-tetrahydroxypentyl residue.

As used herein, the term “polyalkoxyalkyl” refers to a substituent of the formula alkyl-(alkoxy)n-alkoxy wherein n is an integer from 1 to 10, preferably 1 to 4, and more preferably for some embodiments 1 to 3.

Amino acids include glycine, alanine, valine, leucine, isoleucine, methionine, proline, phenylalanine, tryptophan, asparagine, glutamine, glycine, serine, threonine, serine, asparagine, glutamine, tyrosine, cysteine, lysine, arginine, histidine, aspartic acid, glutamic acid, selenocysteine and pyrrolysine. As used herein, reference to “a side chain residue of a natural α-amino acid” specifically includes the side chains of the above-referenced amino acids.

As to any of the groups described herein that contain one or more substituents, it is understood that such groups do not contain any substitution or substitution patterns which are sterically impractical and/or synthetically non-feasible. In addition, the compounds of this invention include all stereochemical isomers arising from the substitution of these compounds. Optional substitution of alkyl groups includes substitution with one or more alkenyl groups, aryl groups or both, wherein the alkenyl groups or aryl groups are optionally substituted. Optional substitution of alkenyl groups includes substitution with one or more alkyl groups, aryl groups, or both, wherein the alkyl groups or aryl groups are optionally substituted. Optional substitution of aryl groups includes substitution of the aryl ring with one or more alkyl groups, alkenyl groups, or both, wherein the alkyl groups or alkenyl groups are optionally substituted.

Optional substituents for any alkyl, alkenyl and aryl group includes substitution with one or more of the following substituents, among others:

halogen, including fluorine, chlorine, bromine or iodine;

pseudohalides, including —CN;

—COOR, where R is a hydrogen or an alkyl group or an aryl group and more specifically where R is a methyl, ethyl, propyl, butyl, or phenyl group all of which groups are optionally substituted;

—COR, where R is a hydrogen or an alkyl group or an aryl group and more specifically where R is a methyl, ethyl, propyl, butyl, or phenyl group all of which groups are optionally substituted;

—CON(R) 2 , where each R, independently of each other R, is a hydrogen or an alkyl group or an aryl group and more specifically where R is a methyl, ethyl, propyl, butyl, or phenyl group all of which groups are optionally substituted; and where R and R can form a ring which can contain one or more double bonds and can contain one or more additional carbon atoms;

—OCON(R) 2 , where each R, independently of each other R, is a hydrogen or an alkyl group or an aryl group and more specifically where R is a methyl, ethyl, propyl, butyl, or phenyl group all of which groups are optionally substituted; and where R and R can form a ring which can contain one or more double bonds and can contain one or more additional carbon atoms;

—N(R) 2 , where each R, independently of each other R, is a hydrogen, or an alkyl group, or an acyl group or an aryl group and more specifically where R is a methyl, ethyl, propyl, butyl, phenyl or acetyl group, all of which are optionally substituted; and where R and R can form a ring that can contain one or more double bonds and can contain one or more additional carbon atoms;

—SR, where R is hydrogen or an alkyl group or an aryl group and more specifically where R is hydrogen, methyl, ethyl, propyl, butyl, or a phenyl group, which are optionally substituted;

—SO 2 R, or —SOR, where R is an alkyl group or an aryl group and more specifically where R is a methyl, ethyl, propyl, butyl, or phenyl group, all of which are optionally substituted;

—OCOOR, where R is an alkyl group or an aryl group;

—SO 2 N(R) 2 , where each R, independently of each other R, is a hydrogen, or an alkyl group, or an aryl group all of which are optionally substituted and wherein R and R can form a ring that can contain one or more double bonds and can contain one or more additional carbon atoms;

—OR, where R is H, an alkyl group, an aryl group, or an acyl group all of which are optionally substituted. In a particular example R can be an acyl yielding —OCOR″, wherein R″ is a hydrogen or an alkyl group or an aryl group and more specifically where R″ is methyl, ethyl, propyl, butyl, or phenyl groups all of which groups are optionally substituted.

Specific substituted alkyl groups include haloalkyl groups, particularly trihalomethyl groups and specifically trifluoromethyl groups. Specific substituted aryl groups include mono-, di-, tri, tetra- and pentahalo-substituted phenyl groups; mono-, di-, tri-, tetra-, penta-, hexa-, and hepta-halo-substituted naphthalene groups; 3- or 4-halo-substituted phenyl groups, 3- or 4-alkyl-substituted phenyl groups, 3- or 4-alkoxy-substituted phenyl groups, 3- or 4-RCO-substituted phenyl, 5- or 6-halo-substituted naphthalene groups. More specifically, substituted aryl groups include acetylphenyl groups, particularly 4-acetylphenyl groups; fluorophenyl groups, particularly 3-fluorophenyl and 4-fluorophenyl groups; chlorophenyl groups, particularly 3-chlorophenyl and 4-chlorophenyl groups; methylphenyl groups, particularly 4-methylphenyl groups; and methoxyphenyl groups, particularly 4-methoxyphenyl groups.

As to any of the above groups that contain one or more substituents, it is understood that such groups do not contain any substitution or substitution patterns which are sterically impractical and/or synthetically non-feasible. In addition, the compounds of this invention include all stereochemical isomers arising from the substitution of these compounds.

The compounds of this invention can contain one or more chiral centers. Accordingly, this invention is intended to include racemic mixtures, diasteromers, enantiomers, tautomers and mixtures enriched in one or more stereoisomer. The scope of the invention as described and claimed encompasses the racemic forms of the compounds as well as the individual enantiomers and non-racemic mixtures thereof.

In general, the terms and phrases used herein have their art-recognized meaning, which can be found by reference to standard texts, journal references and contexts known to those skilled in the art. The following definitions are provided to clarify their specific use in the context of the invention.

Reduction generally refers to a process involving the gain of one or more electrons or a decrease in the oxidation state of an atom or group of atoms in a molecule or ion. Reduction may refer to a reduction half reaction in an oxidation-reduction reaction (i.e., Redox reaction). Reduction may involve reaction of a reactant undergoing reduction with a reductant (also referred to as reducing agent), thereby resulting in a reduction product having one or more atoms characterized by a lower oxidation state as compared to the reactant undergoing reduction. In an embodiment, for example, reduction of molecular nitrogen (N 2 ) results in generation of an ammonia (NH 3 ) or hydrazine (N 2 H 4 ) reduction product.

“Reduction product” refers to a reaction product of a reduction reaction. A reduction product may be characterized by one or more atoms having a lower oxidation state as compared to that of a reactant undergoing reduction. For example, molecular nitrogen (N 2 ) can be reduced to form the reduction product ammonia (NH 3 ) or hydrazine (N 2 H 4 ).

“Metal complex” refers to a composition comprising one or more metal atoms or ions bound to one or more ligands. In an embodiment, metal complex refers to a coordination complex comprising one or more transition metal atom, such as Fe or Co, bound to one or more phosphine ligand, and optionally one or more other groups. The invention includes metal complexes comprising a transition metal atom selected from the group consisting of Fe and Co and a phosphine ligand (L), and optionally a N 2 group. Metal complexes may be neutral or ionically charged, for example, having a charge of +2, +1, −1 or −2.

“Catalyst” refers to a composition that increases the rate of one or more chemical reactions of one or more reactants and is not consumed in the chemical reaction. In an embodiment, for example, a catalyst lowers the activation energy of a chemical reaction, thereby requiring less energy to achieve the transition state(s). Catalysts may participate in multiple chemical transformations in a reaction sequence, thereby increasing the rate of on overall transformation of reactants to products. In an embodiment, the invention provides catalysts for reduction of molecular nitrogen, for example, transition metal catalysts comprising an iron or cobalt complex.

This invention is further explained with the following embodiments, which are not intended to limit the scope of this invention.

FIG. 1A provides a schematic overview of a catalytic cycle for reduction of molecular nitrogen (N 2 ) to ammonia (NH 3 ) involving a transition metal catalyst of the invention. In FIG. 1A , the symbol “Fe” is used as a short hand representation of an iron coordination complex of the invention comprising an iron atom and a phosphine ligand (L). Thus, in FIG. 1A , “Fe” is not merely depicting an iron atom but rather designates an Fe complex of the invention. As shown in FIG. 1 , the dinitrogen adduct (Fe—N 2 ) − under goes protonation and reduction reactions to generate various hydrogenated metal complex intermediates prior to release of a NH 3 reduction product.

FIG. 1B provides a schematic overview of a catalytic cycle for reduction of molecular nitrogen (N 2 ) to ammonia (NH 3 ) involving the specific metal complex (TPB)Fe(N 2 ) − , which comprises an iron atom bound to a tris(phosphinoaryl)borane ligand and a N 2 group. As shown in FIG. 1B , the (TPB)Fe(N 2 ) catalyst participates in protonation and reduction reactions in the presence of 1 atmosphere of N 2 resulting in generation of a NH 3 reduction product.

(1) Smil, V. Enriching the Earth . Cambridge, Mass., MIT Press (2001).

(2) Burgess, B. K. & Lowe, D. J. Mechanism of molybdenum nitrogenase. Chem. Rev. 96, 2983-3011 (1996).

(3) Eady, R. R. Structure-function relationships of alternative nitrogenases. Chem. Rev. 96, 3013-3030 (1996).

(4) Howard, J. B. & Rees, D. C. How many metals does it take to fix N 2 ? A mechanistic overview of biological nitrogen fixation. Proc. Natl. Acad. Sci. U.S.A. 103, 17088-17093 (2006).

(5) Schwarz, G., Mendel R. R. & Ribbe, M. W. Molybdenum cofactors, enzymes and pathways. Nature 460, 839-847 (2009)

(6) Seefeldt, L., Hoffman, B. M. & Dean, D. R. Mechanism of Mo-dependent nitrogenase. Annu. Rev. Biochem., 78, 701-722 (2009)

(7) Peters, J. C. & Mehn, M. P. in Bio - organometallic Approaches to Nitrogen Fixation Chemistry Activation of Small Molecules , Tolman, W. B. Ed. (Wiley-VCH, 2006) pp. 81-119.

(8) Shilov, A. E. et al. New nitrogenase model for reduction of molecular nitrogen in protonic media. Nature 231, 460-461 (1971).

(9) Chatt, J., Dilworth, J. R. & Richards, R. L. Recent advances in chemistry of nitrogen-fixation. Chem. Rev. 78, 589-625 (1978).

(10) Yandulov, D. V. & Schrock R. R. Catalytic reduction of dinitrogen to ammonia at a single molybdenum center. Science 301, 76-78 (2003).

(11) Arashiba, A., Miyake, Y. & Nishibayashi, Y. A molybdenum complex bearing PNP-type pincer ligands leads to the catalytic reduction of dinitrogen into ammonia. Nature Chem. 3, 120-125 (2011).

(12) Hoffman, B. M., Dean, D. R. & Seefeldt, L. C. Climbing nitrogenase: toward a mechanism of enzymatic nitrogen fixation. Acc. Chem. Res. 42, 609-619 (2009).

(13) Moret, M.-E. & Peters, J. C. N 2 functionalization at iron metallaboratranes. J. Am. Chem. Soc. 133, 18118-18121 (2011).

(14) Moret, M.-E. & Peters, J. C. Terminal iron dinitrogen and iron imide complexes supported by a tris(phosphino)borane ligand. Angew. Chem. Int. Ed. 50, 2063-2067 (2011).

(15) Anderson, J. A., Moret, M.-E. & Peters, J. C. Conversion of Fe—NH 2 to Fe—N 2 with release of NH 3 . J. Am. Chem. Soc. 135, 534-537 (2013).

(16) K. M. Lancaster et al. X-ray emission spectroscopy evidences a central carbon in the nitrogenase iron-molybdenum cofactor. Science 334, 974-977 (2011).

(17) T. M. Spatzal et al. Evidence for interstitial carbon in nitrogenase FeMo cofactor. Science 334, 940-940 (2011).

(18) M. P. Hendrich et al. On the feasibility of N 2 fixation via a single-site Fe I /Fe IV cycle: Spectroscopic studies of Fe I (N 2 )Fe I , Fe IV ≡N, and related species. Proc. Natl. Acad. Sci. U.S.A. 103, 17107-17112 (2006).

(19) Pickett, C. J. & Talarmin J. Electrosynthesis of ammonia. Nature 317, 652-653 (1985).

(20) Fryzuk, M. D., Love, J. B., Rettig, S. J. & Young, V. G. Transformation of coordinated dinitrogen by reaction with dihydrogen and primary silanes. Science 275, 1445-1447 (1997).

(21) Pool, J. A., Lobkovsky, E. & Chirik, P. J. Hydrogenation and cleavage of dinitrogen to ammonia with a zirconium complex. Nature 427, 527-530 (2004).

(22) Rodriguez, M. M., Bill, E., Brennessel, W. W. & Holland, P. L. N 2 reduction and hydrogenation to ammonia by a molecular iron-potassium complex. Science 334, 780-783 (2011).

(23) Hills, A. et al. Bis[1,2-bis(dimethylphosphino)ethane]dihydrogenhydridoiron(II) tetraphenylborate as a model for the function of nitrogenases. J. Chem. Soc. Dalton Trans. 25, 3041-3049 (1993).

(24) Crossland, J. L. & Tyler, D. R. Iron-dinitrogen coordination chemistry: Dinitrogen activation and reactivity. Coord. Chem. Rev. 255, 949-974 (2011).

(25) Yuki, M., Tanaka, H., Miyake, Y., Yoshizawa, K. & Nishibayashi, Y. Iron-catalysed transformation of molecular dinitrogen into silylamine under ambient conditions. Nature Comm. 3, 1254 (2012).

(26) Lee, Y. H., Mankad, N. P. & Peters, J. C. Triggering N 2 uptake via redox-induced expulsion of coordinated NH 3 and N 2 silylation at trigonal bipyramidal iron. Nature Chem. 2, 558-565 (2010).

(27) Bell, L. G. & Brintzinger, H. H. Stoichiometric reduction of molecular nitrogen by iron complexes. J. Am. Chem. Soc. 92, 4464-4465 (1970).

(28) D. A. Hall, G. J. Leigh, J. Chem. Soc., Dalton, 3539-3541 (1996).

(29) Fong, H., Moret, M.-E., Lee, Y. H. & Peters, J. C. Heterolytic H 2 cleavage and catalytic hydrogenation by an iron metallaboratrane. Organometallics 32, 3053-3062 (2013).

(30) Schrock, R. R. Nitrogen reduction: molybdenum does it again. Nature Chem. 3, 95-96 (2011).

(31) Yandulov, D. V. & Schrock, R. R. Reduction of dinitrogen to ammonia at a well-protected reaction site in a molybdenum triamidoamine complex. J. Am. Chem. Soc. 124, 6252-6253 (2002).

(32) Brookhart, M., Grant, B. & Volpe Jr., A. F. [(3,5-(CF 3 ) 2 C 6 H 3 ) 4 B] − [H(OEt 2 ) 2 ] + : a convenient reagent for generation and stabilization of cationic, highly electrophilic organometallic complexes. Organometallics 11, 3920-3922 (1992).

(33) Kern, R. J. Tetrahydrofuran complexes of transition metal chlorides. J. Inorg. Nucl. Chem. 24, 1105-1109 (1962).

(34) Wietz, I. S. & Rabinovitz, M. The application of C 8 K for organic synthesis: reduction of substituted naphthalenes. J. Chem. Soc., Perkin Trans. 1, 117-120 (1993).

(35) Komiya, S. et al., Isolation of a zerovalent iron dinitrogen complex with 1,2-bis(diethylphosphino)ethane ligands. J. Chem. Soc., Chem. Commun. 1993, 787-788.

(36) Weatherburn, M. W. Phenol-hypochlorite reaction for determination of ammonia. Anal. Chem. 39, 971 (1967).

(37) Watt, G. W. & Chrisp, J. D. A Spectrophotometric Method for the Determination of Hydrazine. Anal. Chem. 24, 2006 (1952).

General Considerations

Unless otherwise noted, all compounds were purchased from commercial sources and used without further purification. [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ], 14 [(TPB)Fe][BAr F 4 ], 15 (TPB)(μ-H)Fe(H)(N 2 ), 29 [Lutidinium][BAr F 4 ], 31 HBAr F 4 .2 Et 2 O, 32 [(SiP iPr 3 )Fe(N 2 )][Na(12-crown-4) 2 ], 26 FeCl 2 .1.5 THF, 33 KC 8 , 34 [(TPB)Fe(NH 3 )][BAr F 4 ], 15 [(TPB)Fe(N 2 H 4 )][BAr F 4 ], 15 (TPB)Fe≡N(p-C 6 H 4 OMe), 14 and Fe(depe) 2 N 2 35 were prepared according to literature procedures ([Lutidinium]=2,6-dimethylpyridinium, [BAr F 4 ]=[B(3,5-(CF 3 ) 2 —C 6 H 3 ) 4 B] − ). All manipulations were carried out under an N 2 atmosphere utilizing standard glovebox or Schlenk techniques. Solvents were dried and de-oxygenated by an argon sparge followed by passage through an activated alumina column purchased from S.G. Waters Company. Labeled 15 N 2 (98% purity) was obtained from Cambridge Isotope Laboratories. Solvents for catalytic runs were additionally stirred for more than 2 hours over Na/K alloy then filtered prior to use.

IR Spectroscopy

IR spectra were obtained via KBr pellets on a Bio-Rad Excalibur FTS 3000 spectrometer using Varian Resolutions Pro software set at 4 cm −1 resolution.

NMR Spectroscopy

NMR measurements were obtained on Varian 300 MHz or 500 MHz spectrometers. Deuterated solvents for these measurements were obtained from Cambridge Isotope Laboratories and were dried and degassed prior to use. All 1 H NMR spectra were referenced to residual solvent peaks.

EPR Spectroscopy

EPR X-band spectra were obtained on a Bruker EMX spectrometer with the aid of Bruker Win-EPR software suite version 3.0. The spectrometer was equipped with a rectangular cavity which operated in the TE 102 mode. Temperature control was achieved with a liquid N 2 filled quartz dewar in which the sample was submerged during data collection.

UV-Visible Spectroscopy

UV-Visible spectra were taken on a Cary 50 spectrometer from 1100 nm to 200 nm in the fast scan mode. Samples were prepared in a 1 cm path length quartz cuvette. All samples had a blank sample background subtraction applied.

Standard Catalytic Protocol

[(TPB)Fe(N 2 )][Na(12-crown-4) 2 ] (2 mg, 0.002 mmol) was suspended in Et 2 O (0.5 mL) in a 20 mL scintillation vial equipped with a stir bar. This suspension was vigorously stirred and cooled to −78° C. in a cold well inside of the glove box. A similarly cooled solution of HBAr F 4 .2 Et 2 O (93 mg, 0.092 mmol) in Et 2 O (1.5 mL) was added to the suspension in one portion with rapid stirring. Any remaining acid was dissolved in cold Et 2 O (0.25 mL) and added subsequently. The reaction mixture turned light yellow-orange and homogeneous upon addition of acid and the resulting solution was allowed to stir for 5 minutes before being transferred into a pre-cooled Schlenk tube equipped with a stir bar. The original reaction vial was washed with cold Et 2 O (0.25 mL) which was subsequently transferred to the Schlenk tube. Solid KC 8 (15 mg, 0.100 mmol) was suspended in cold Et 2 O (0.75 mL) and added dropwise to the rapidly stirred solution in the Schlenk tube which was then tightly sealed. The reaction was allowed to stir for 40 minutes at −78° C. before being warmed to room temperature and stirred for 15 minutes.

Ammonia Quantification

A Schlenk tube was charged with HCl (3 mL of a 2.0 M solution in Et 2 O, 6 mmol). Reaction mixtures were vacuum transferred into this collection flask. Residual solid in the reaction vessel was treated with a solution of [Na][O-t-Bu] (40 mg, 0.4 mmol) in 1,2-dimethoxyethane (1 mL) and sealed. The resulting suspension was allowed to stir for 10 minutes before all volatiles were again vacuum transferred into the collection flask. After completion of the vacuum transfer, the flask was sealed and warmed to room temperature. Solvent was removed in vacuo and the remaining residue was dissolved in H 2 O (1 mL). An aliquot of this solution (20 or 40 μL) was then analyzed for the presence of NH 3 (trapped as [NH 4 ][Cl]) via the indophenol method. 36 Quantification was performed with UV-Visible spectroscopy by analyzing the absorbance at 635 nm. The tables shown below list the raw data for the runs. Runs with small absorbance levels (<0.02 absorbance units) suffer from a large degree of error due to a small signal-to-noise ratio. Catalytic runs that used a 40 μL aliquot are denoted with an asterisk, accounting for larger relative absorbances.

Catalytic Protocol Under 15 N 2

[(TPB)Fe(N 2 )][Na(12-crown-4) 2 ] (4 mg, 0.004 mmol) was suspended in Et 2 O (3 mL) in a 25 mL three neck flask (ground-glass, 14/20) equipped with a stir bar. The flask was then equipped with a stopcock adaptor in the central opening, a solid addition arm containing HBAr F 4 .2 Et 2 O (188 mg, 0.186 mmol) in one of the side openings, and an additional solid addition arm containing KC 8 (37 mg, 0.274 mmol) in the final opening. The apparatus was sealed, brought out of the glovebox, and connected to a high-vacuum manifold. The solution was degassed via four freeze-pump-thaw cycles and then allowed to thaw to −78° C. with stirring. The flask was backfilled with 1 atm 15 N 2 . At this point the acid was added to the solution in one portion via the solid addition arm. The resulting solution was allowed to stir for 5 minutes before KC 8 was added via the other solid addition arm resulting in a dark suspension. This suspension was allowed to stir for 40 minutes at −78° C. and then an additional 10 minutes at room temperature prior to the standard work-up. The presence of [ 15 NH 4 ][Cl] was verified by 1 H NMR spectroscopy ( FIG. 8 ). The yield of NH 3 was 3.64 equiv NH 3 /Fe as measured using the indophenol method. The NH 3 yield, while still showing catalysis, was lower than the average obtained by the standard protocol, presumably due to differences associated with adding the HBAr F 4 .2 Et 2 O and KC 8 solids via the solid addition arms.

Runs with [(TPB)Fe][BAr F 4 ] as Precursor

The procedure was identical to that of the standard catalytic protocol with the changes noted. The precursor used was [(TPB)Fe][BAr F 4 ] (2.3 mg, 0.002 mmol) which is a dark orange solid. Note that [(TPB)Fe][BAr F 4 ] was soluble in Et 2 O and formed a yellow solution. No substantial color change was observed upon addition of acid.

Runs with [(SiP iPr 3 )Fe(N 2 )][Na(12-crown-4) 2 ] as Precursor

The procedure was identical to that of the standard catalytic protocol with the changes noted. The precursor used was [(SiP iPr 3 )Fe(N 2 )][Na(12-crown-4) 2 ] (2 mg, 0.002 mmol) which is a dark purple solid.

Runs with (TPB)(μ-H)Fe(H)(N 2 ) as Precursor

The procedure was identical to that of the standard catalytic protocol with the changes noted. The precursor used was (TPB)(μ-H)Fe(H)(N 2 ) (1.3 mg, 0.002 mmol) which is a yellow solid. Note that (TPB)(μ-H)Fe(H)(N 2 ) was insoluble in Et 2 O and did not dissolve upon addition of acid. As such, the resulting mixture was a suspension through the remaining manipulations.

Runs with FeCl 2 .1.5 THF as Precursor

The procedure was identical to that of the standard catalytic protocol with the changes noted. The precursor used was FeCl 2 .1.5 THF (0.5 mg, 0.002 mmol) which is an off white powder. Note that FeCl 2 .1.5 THF did not dissolve upon addition of acid. As such, the resulting mixture was a suspension through the remaining manipulations.

Runs with FeCl 3 as Precursor

The procedure was identical to that of the standard catalytic protocol with the changes noted. The precursor used was FeCl 3 (0.3 mg, 0.002 mmol) which is a dark solid. Note that FeCl 3 was soluble in Et 2 O and formed a yellow solution. No substantial color change was observed upon addition of acid.

Runs with Fe(CO) 5 as Precursor

The procedure was identical to that of the standard catalytic protocol with the changes noted. The precursor used was Fe(CO) 5 (0.35 mg, 0.002 mmol) which is a pale yellow liquid. Note that Fe(CO) 5 was soluble in Et 2 O and formed a colorless solution. No substantial color change was observed upon addition of acid.

Runs with FeCp 2 as Precursor

The procedure was identical to that of the standard catalytic protocol with the changes noted. The precursor used was FeCp 2 (0.35 mg, 0.002 mmol) which is an orange solid. Note that FeCp 2 was soluble in Et 2 O and formed a yellow solution. No substantial color change was observed upon addition of acid.

Runs without an Fe Precursor

The procedure was identical to that of the standard catalytic protocol with the changes noted. A 2 mL Et 2 O solution of HBAr F 4 .2 Et 2 O (93 mg, 0.092 mmol) was added directly into a Schlenk tube equipped with a stir bar and cooled to −78° C. Addition of KC 8 and subsequent work-up was identical to the standard catalytic protocol.

Runs with [Lutidinium][BAr F 4 ] as Acid

The procedure was identical to that of the standard catalytic protocol with the changes noted. The acid used was [Lutidinium][BAr F 4 ] (0.090 g, 0.092 mmol).

Runs with HCl as Acid

The procedure was identical to that of the standard catalytic protocol with the changes noted. The acid used was anhydrous HCl (46 μL of a 2.0 M solution in Et 2 O, 0.092 mmol) which was added without further dilution. The red suspension turned light yellow upon addition of acid, and subsequently precipitated a fine yellow solid. All subsequent manipulations were carried out with this suspension.

Runs with Trifluoromethanesulfonic Acid as Acid

The procedure was identical to that of the standard catalytic protocol with the changes noted. The acid used was trifluoromethanesulfonic acid (131 μL of a 0.7 M solution in Et 2 O, 0.092 mmol) which was added without further dilution. The red suspension turned light yellow-green and homogenized upon addition of acid.

Runs with K as Reductant

The procedure was identical to that of the standard catalytic protocol with the changes noted. The reductant used was K metal (4 mg, 0.1 mmol) which was added as a solid. The reaction mixture was allowed to stir at −78° C. for 40 minutes and was then warmed slowly to RT overnight. After this time, a pale red-orange solution was present. Longer reaction times were employed to ameliorate the effect of the small surface area of the K metal.

Runs with CoCp* 2 as Reductant

The procedure was identical to that of the standard catalytic protocol with the changes noted. The reductant used was decamethylcobaltocene, CoCp* 2 , (19 mg, 0.058 mmol) which was added as a solid. A heterogeneous mixture resulted at −78° C. that homogenized at room temperature, producing a yellow solution.

Runs with CrCp* 2 as Reductant

The procedure was identical to that of the standard catalytic protocol with the changes noted. The reductant used was decamethylchromocene, CrCp* 2 , (20 mg, 0.062 mmol) which was added as a solid. The resulting suspension darkened before gradually returning to a yellow color.

Supplemental Discussion

In addition to the standard precatalyst [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ] and the cationic complex [(TPB)Fe][BAr F 4 ], we examined the related TPB-containing complexes [(TPB)Fe(NH 3 )][BAr F 4 ] 15 and [(TPB)Fe(N 2 H 4 )][BAr F 4 ] 15 as precatalysts for NH 3 production using the standard catalytic conditions. The modest attenuation in NH 3 yields (see below) relative to the yields when [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ] is used as the precatalyst may reflect less than quantitative cycling of the cationic derivatives to the Fe-bound N 2 species.

Runs with [(TPB)Fe(NH 3 )][BAr F 4 ] as Precursor

The procedure was identical to that of the standard catalytic protocol with the changes noted. The precursor used was [(TPB)Fe(NH 3 )][BAr F 4 ] (2.9 mg, 0.002 mmol) which is an orange solid. Note that the solution became homogeneous with no significant color change upon addition of acid.

Runs with [(TPB)Fe(N 2 H 4 )][BAr F 4 ] as Precursor

The procedure was identical to that of the standard catalytic protocol with the changes noted. The precursor used was [(TPB)Fe(N 2 H 4 )][BAr F 4 ] (2.9 mg, 0.002 mmol) which is an orange solid. Note that the solution became homogeneous with no significant color change upon addition of acid.

Runs with Fe(depe) 2 N 2 as Precursor

The procedure was identical to that of the standard catalytic protocol with the changes noted. The precursor used was Fe(depe) 2 N 2 (1 mg, 0.002 mmol), which is a dark red solid. Note that the solution became homogeneous with no significant color change upon addition of acid.

Runs at Room Temperature

The procedure was identical to that of the standard catalytic protocol with the changes noted. All manipulations were performed analogously to the standard conditions at room temperature.

Supplemental Discussion

Hydrazine (N 2 H 4 ) is a possible product of the N 2 reduction catalysis described but is not detected under the standard catalytic protocol using [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ] as the precatalyst. As the experiment below establishes, even if N 2 H 4 is produced as an intermediate en route to NH 3 formation, it would be likely be consumed and converted to NH 3 under the standard catalysis conditions employed.

Inclusion of Hydrazine in a Catalytic Run with [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ]

The procedure was identical to that of the standard catalytic protocol with the following changes noted. Hydrazine (0.12 μL, 0.004 mmol) was added directly to the suspension of [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ] prior to subjecting the mixture to the standard catalytic protocol. No color change was observed upon addition of N 2 H 4 . After workup the aqueous solution was analyzed for NH 3 as described above, and also for N 2 H 4 via a literature protocol. 37 The results obtained show most all of the N 2 H 4 had been consumed, indicating that if it is produced in some amount under the standard catalytic protocol it is unlikely to be detectable. As a control experiment it was shown that N 2 H 4 was not degraded to NH 3 under the standard catalytic conditions in the absence of [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ].

IR Spectral Analysis of Addition of 2 Equiv HBAr F 4 .2 Et 2 O to [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ], Followed by 3 Equiv KC 8

A 20 mL scintillation vial was charged with a stir bar and [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ] (8 mg, 0.0074 mmol). In a separate vial, HBAr F 4 .2 Et 2 O (15 mg, 0.015 mmol) was dissolved in Et 2 O (1 mL). Finally, a third vial was prepared containing a suspension of potassium graphite (3 mg, 0.023 mmol) in Et 2 O (1 mL). All three vials were chilled in the cold well to −70+/−5° C. for 30 minutes. The solution of HBAr F 4 .2 Et 2 O was quickly added to the stirring suspension of [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ] with a glass pipette pre-cooled to −70° C. Any residue of the acid was washed with pre-chilled Et 2 O (0.5 mL) and transferred to the stirring solution. The resulting solution turned homogeneous. After stirring for 5 minutes, the suspension of KC 8 was added rapidly to the stirring solution. Any additional KC 8 was washed with pre-chilled Et 2 O (0.5 mL) and the resulting suspension was transferred to the stirring mixture. After addition of KC 8 the solution adopted a red color. This mixture was capped and stirred at −70° C. for 40 minutes and then brought to room temperature and stirred for 10 minutes. The red color persisted upon thawing to room temperature. Graphite was removed by filtration through glass filter paper. To the red solution was added 12-crown-4 (13.1 mg, 74.3 μmol) in Et 2 O (1 mL) and the resulting solution was stirred for 10 minutes. The solution was then cooled to −70° C. for 30 minutes and stirred vigorously, leading to a red precipitate. The precipitate was collected on a filter pad and the light orange filtrate was concentrated to dryness in vacuo. IR analysis of the precipitate (KBr pellet) showed an intense band at v NN =1904 cm −1 , identical to that of authentic [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ] (v NN =1905 cm −1 , KBr pellet). No assignable v NN IR bands were observed for the filtrate in the window of 1700-2300 cm −1 . See FIG. 10 .

IR and 31 P NMR Spectral Analysis of Addition of 10 Equiv HBAr F 4 .2 Et 2 O to [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ], Followed by 12 Equiv KC 8

A 20 mL scintillation vial was charged with a stir bar and [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ] (10.4 mg, 10.2 μmol). In a separate vial, HBAr F 4 .2 Et 2 O (103 mg, 102 μmol) was dissolved in Et 2 O (1 mL). Finally, a third vial was prepared containing a suspension of potassium graphite (16.5 mg, 122 μmol) in Et 2 O (1 mL). All three vials were chilled in the cold well to −70+/−5° C. for 30 minutes. The solution of HBAr F 4 .2 Et 2 O was quickly added to the stirring suspension of [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ] with a glass pipette pre-cooled to −70° C. Any residue of the acid was washed with pre-chilled Et 2 O (0.5 mL) and transferred to the stirring solution. The resulting solution turned homogeneous. After stirring for 5 minutes, the suspension of KC 8 was added rapidly to the stirring solution. Any additional KC 8 was washed with pre-chilled Et 2 O (0.5 mL) and the resulting suspension was transferred to the stirring mixture. The large amount of graphite present in the vial prevented the color of the resulting solution to be accurately discerned. This mixture was capped and stirred at −70° C. for 40 minutes and then brought to RT and stirred for 10 minutes. Graphite was removed by filtration through glass filter paper. To the resulting orange solution was added 12-crown-4 (60 mg, 340 μmol) in Et 2 O (1 mL) and a 31 P NMR integration standard of triphenylphosphine (11.9 mg, 45.4 μmol) in toluene (1 mL) followed by stirring for 10 minutes. The solution was then cooled to −70° C. for 30 minutes and stirred vigorously. No precipitate formed and volatiles were removed in vacuo. The orange powder was dissolved in THF and integration of 31 P NMR resonances suggest the formation of (TPB)(μ-H)Fe(N 2 )(H) (3.4 μmol) in 30% yield. Solid-state IR analysis of the orange solid (KBr pellet) showed a strong, sharp band at v NN =2073 cm −1 (s), identical to that of authentic (TPB)(μ-H)Fe(N 2 )(H). Additional broad, weak bands were observed at 1942, 1875, 1802, 1734 cm −1 that could not be assigned. 31 P{ 1 H} NMR (400 MHz, THF): 72.6, 63.1 ppm. See FIGS. 10 and 11 .

Reactivity of (TPB)(μ-H)Fe(N 2 )(H) with KC 8

A 20 mL scintillation vial was charged with a stir bar and (TPB)(μ-H)Fe(N 2 )(H) (11 mg, 0.016 mmol) suspended in Et 2 O (2 mL). A separate vial was charged with KC 8 (2.6 mg, 0.019 mmol) suspended in Et 2 O (2 mL). Both vials were cooled to −70+/−5° C. and the Fe-containing vial was stirred vigorously. The suspension of KC 8 was quickly transferred to the vial containing (TPB)(μ-H)Fe(N 2 )(H) and stirred for 10 minutes at low temperature. The vial was then brought to room temperature and the brown color of KC 8 slowly turned to black over 1 hour. Graphite was filtered through a glass filter pad and the orange filtrate was transferred to a vial containing 12-crown-4 (21.0 mg, 119.17 μmol, 7.25 equivalents) and stirred vigorously at −70° C. for 10 minutes. No precipitate formed and the resulting orange solution was brought to room temperature and concentrated to dryness in vacuo. IR analysis of the residue (KBr pellet) showed a strong stretch at v NN =2073 cm −1 , consistent with authentic (TPB)(μ-H)Fe(N 2 )(H) (2073 cm −1 , KBr pellet). 1 H NMR analysis was consistent with the presence of predominately (TPB)(μ-H)Fe(N 2 )(H) and minor amounts of unidentified paramagnetic species.

Reactivity of (TPB)(μ-H)Fe(N 2 )(H) with HBAr F 4 .2 Et 2 O

A 20 mL scintillation vial was charged with a stir bar and (TPB)(μ-H)Fe(N 2 )(H) (9 mg, 0.014 mmol) suspended in Et 2 O (2 mL). A separate vial was charged with HBAr F 4 .2 Et 2 O (15 mg, 0.015 mmol, 1.08) suspended in Et 2 O (2 mL). Both vials were cooled to −70+/−5° C. and the Fe-containing vial was stirred vigorously. The solution of HBAr F 4 . 2 Et 2 O was quickly transferred to the vial containing (TPB)(μ-H)Fe(N 2 )(H) and stirred for 10 minutes at low temperature. The vial was then brought to room temperature and no noticeable color change was observed over 1 hour. The solution was concentrated to dryness in vacuo and the remaining residue was analyzed with IR spectroscopy (KBr pellet) which showed a strong stretch at v NN =2073 cm −1 , consistent with authentic (TPB)(μ-H)Fe(N 2 )(H) (2073 cm −1 , KBr pellet). The residue was then re-dissolved in C 6 D 6 and analyzed by 1 H NMR spectroscopy which showed (TPB)(μ-H)Fe(N 2 )(H) with minor amounts of unidentified paramagnetic species and resonances from the BAr F 4 anion. Complete consumption of (TPB)(μ-H)Fe(N 2 )(H) to unidentified paramagnetic species was observed after 12 hours at room temperature.

Identification of [(TPB)Fe(NH 3 )][BAr F 4 ] from Protonation of [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ]

[(TPB)Fe(N 2 )][Na(12-crown-4) 2 ] (5 mg, 0.005 mmol) was dissolved in 2 mL of THF and cooled to −78° C. This dark red solution was added dropwise to a similarly cooled 2 mL THF solution of HBAr F 4 .2 Et 2 O (29 mg, 0.029 mmol) with stirring. The resulting yellow-orange solution was allowed to stir for 10 minutes at low temperature before being warmed to room temperature and stirred for an additional 40 minutes. 1,8-Bis(dimethylamino)naphthalene (6 mg, 0.029 mmol) was added and the solution was allowed to stir for 15 minutes with no noticeable color change. Volatiles were removed from the solution and the resulting yellow residue was taken up in THF-d 8 . The presence of [(TPB)Fe(NH 3 )][BAr F 4 ] 15 was determined by comparison of the 1 H NMR spectrum with that of an authentic sample prepared as recently reported. Additionally, a capillary insert of the (TPB)FeMe 15 in THF-d 8 was added to the NMR sample which allowed for crude measurements of the yield of [(TPB)Fe(NH 3 )][BAr F 4 ], a species tentatively assigned as [(TPB)Fe][BAr F 4 ] 15 and the total amount of S=3/2 TPB species as roughly 30%, 50%, and 100% respectively. Note that there is likely a significant degree of error on these measurements due to the broad paramagnetic peaks used for integration. See FIG. 5 .

Identification of H 2 in Standard Catalytic Runs

The catalytic runs were performed according to the standard procedure. Prior to the vacuum transfer of volatiles, the solutions inside of the Schlenk tubes were frozen. The ground glass joint of the Schlenk tube was then sealed with a rubber septum and the head space between the Teflon stopcock of the Schlenk tube and the septum was evacuated. This head space was left under static vacuum and the Teflon stopcock of the reaction vessel was opened after which a 10 mL aliquot of the headspace was sampled through the septa via a gas-tight syringe. This sample was then analyzed for hydrogen with an Agilent 7890A gas chromatograph using a thermal conductivity detector. After H 2 analysis, the reaction vessel was sealed and subjected to the standard analysis for NH 3 . As some H 2 leakage is unavoidable by the procedure used, these values represent lower limits of the H 2 yield.

Identification of H 2 in Runs without an Fe Precursor

A Schlenk tube was charged with a stir bar and a suspension of KC 8 (14 mg, 0.100 mmol) in Et 2 O (0.5 mL). The Schlenk tube was then fitted with a Teflon stopcock, but not sealed. The ground glass joint on the Schlenk tube was sealed with a rubber septum. This reaction vessel was then cooled to −78° C. A pre-cooled solution of HBAr F 4 .2 Et 2 O (92 mg, 0.092 mmol) in Et 2 O (2 mL) was then syringed directly into the reaction vessel with stirring after which the vessel was rapidly sealed with its Teflon stopcock. The reaction was allowed to stir for 40 minutes at low temperature before the headspace between the Teflon valve and the septa was evacuated. After evacuation, the Teflon stopcock was opened and a 10 mL aliquot of the headspace was sampled via a gas tight syringe. This sample was then analyzed for hydrogen with an Agilent 7890A gas chromatograph using a thermal conductivity detector. The yield of hydrogen observed, based on proton-equivalents was 66% and 88% for each of two runs, respectively. As some H 2 leakage is unavoidable by the procedure used, these values represent lower limits of the H 2 yield.

Referring to FIG. 6 , the middle spectrum was generated by dissolving [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ] (4 mg, 0.004 mmol) in 250 μL of 2-MeTHF to generate a deep red solution. This solution was then transferred to an EPR tube and frozen within a liquid N 2 cooled cold well. Another 250 μL 2-MeTHF solution of HBAr F 4 .2 Et 2 O (38 mg, 0.037 mmol) was prepared and carefully layered onto the frozen solution of [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ] in the EPR tube. The layered solutions were then frozen. At this time, the solutions were warmed until barely thawing (−140° C.) and mechanically mixed with a long needle. Upon mixing, the red color of [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ] disappeared and a yellow solution was obtained. The solution was frozen and the EPR spectrum shown was obtained at 77 K. Immediately prior to obtaining this spectrum, a spectrum of a sample of [(TPB)Fe(N 2 )][Na(12-crown-4) 2 ] (4 mg, 0.004 mmol) in 500 μL of 2-MeTHF was obtained under identical conditions. Both spectra were then doubly integrated and compared to obtain an integrated yield for the formation of the new S=½ species shown in the figure. Repetition of this experiment in triplicate provided an average yield of ˜90%.

Referring to FIG. 6 , the bottom spectrum was generated by dissolving (TPB)Fe≡N(p-C 6 H 4 OMe) 14 (3 mg, 0.004 mmol) in 250 μL of 2-MeTHF. This solution was then cooled to −78° C. and then mixed with a similarly cooled 250 μL solution of [Fc][BAr F 4 ] (Fc=ferrocenium) (4 mg, 0.004 mmol) with rapid stirring. Upon mixing the dark blue color of [Fc][BAr F 4 ] disappeared and the dark green color of (TPB)Fe≡N(p-C 6 H 4 OMe) lightened slightly to green-yellow. The solution was allowed to stir for an additional 10 minutes then transferred to a quartz EPR tube and frozen. The EPR spectrum shown was obtained at 77 K. Note that the asterisk denotes a signal of an unknown S=½ component of the product mixture.

To conclude, we have synthetically introduced the tripodal (CPiPr3)H ligand and have prepared and structurally compared its {(CP iPr 3 )FeN 2 } n complexes (n=0, −1, +1) with those of the isostructural series {(SiP iPr 3 )FeN 2 } n . The {(CP iPr 3 )FeN 2 } n complexes feature an axial N 2 ligand bound trans to an axial C-atom in a trigonal bipyramidal geometry, a design meant to model one plausible geometry for a single Fe—N 2 binding site in the iron-molybdenum cofactor (FeMoco). The C alkyl -Fe interaction in the (CP iPr 3 )Fe system exhibits a substantially higher degree of ionic character, and is more flexible, than for the related Si silyl -Fe interaction in the isostructural and isoelectronic (SiP iPr 3 )Fe system. 17 These results support that this type of Fe—C flexibility generally models the flexibility one can intuit for an N 2 —Fe—C interstitial interaction within FeMoco. Whereas the N 2 anion (SiP iPr 3 )FeN 2 − does not effectively facilitate the delivery of H-atoms to N 2 to produce NH 3 via proton/reductant equivalents, an Et 2 O solution of (CP iPr 3 )FeN 2 − under 1 atm of N 2 releases ca. 4.6 equiv NH 3 relative to Fe. The modest catalytic N 2 reduction behavior of (CP iPr 3 )FeN 2 − at −78° C. is comparable to (TP iPr B) FeN 2 − . 21

It is noteworthy that amongst the isostructural SiP iPr 3 , TP iPr B, and CP iPr 3 series, the system with the most flexible axial linkage, (TP iPr B)Fe, gives the greatest catalytic yield under a common set of reaction conditions, while the least flexible, (SiP iPr 3 )Fe, gives only substoichiometric yields of ammonia; the (CP iPr 3 )Fe system falls in between the two both in terms of flexibility and catalytic competence. These results may be consistent with the hypothesis that a flexible Fe—C interstitial interaction might facilitate N 2 binding and reduction at a single Fe site within FeMoco. Our structural and DFT studies 22 demonstrate that, in the right environment, a carbon atom can serve as a modestly flexible ligand trans to an Fe—N 2 binding site, and that this flexibility is enhanced by the ability of the carbon to accommodate a significant ionic charge. It seems likely to us that the inorganic carbide ligand in FeMoco is similarly, and likely more, able to stabilize substantial ionic character in the Fe—C interstitial bond ( FIG. 12 ), resulting in a flexible interaction that initially exposes an N 2 binding site that can be further modulated as a function of the N x H y reduction state.

Within our synthetic series, it may be that different catalysts follow different mechanistic pathways (distal vs. alternating, or some hybrid path); 21,41 for instance the most flexible system, (TP iPr B)Fe, may be better suited to facilitate a distal pathway that samples strongly pi-bonded intermediates, while (CP iPr 3 )Fe, which we presume is less flexible, could instead be dominated by an alternating or hybrid pathway.

Experimental Methods

General. All manipulations were carried out using standard Schlenk or glovebox techniques under an N 2 atmosphere. Unless otherwise noted, solvents were deoxygenated and dried by thoroughly sparging with N 2 followed by passage through an activated alumina column in a solvent purification system by SG Water, USA LLC. Non-halogenated solvents were tested with a standard purple solution of sodium benzophenone ketyl in tetrahydrofuran in order to confirm effective moisture removal. O-iodotriphenylmethane, 28 H(OEt 2 ) 2 [B(3,5-(CF 3 ) 2 —C 6 H 3 ) 4 ], 42 KC 8 , 43 [(TPB)FeN 2 ][Na(12-crown-4) 2 ], 25 [(SiP iPr 3 )FeN 2 ][Na(12-crown-4) 2 ] 17 and [(C Si P Ph 3 )FeN 2 ][K(18-crown-6) 2 ] 22 were prepared according to literature procedures. [Decamethylferrocenium][B(3,5-(CF 3 ) 2 —C 6 H 3 ) 4 ] was prepared by treating [ferrocenium][B(3,5-(CF 3 ) 2 —C 6 H 3 ) 4 ] 44 with decamethylferrocene and used without purification. FeI 2 (THF) 2 was prepared by treating Fe powder with I 2 in THF, 45 and was dried to FeI 2 by heating under vacuum at 80° C. for 6 hours. All other reagents were purchased from commercial vendors and used without further purification unless otherwise stated.

Physical Methods. Elemental analyses were performed by Robinson Microlit Laboratories (Ledgewood, N.J.). Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc., degassed, and dried over active 3-Å molecular sieves prior to use. 1 H and 13 C chemical shifts are reported in ppm relative to tetramethylsilane, using residual proton and 13 C resonances from solvent as internal standards. 31 P and 19 F chemical shifts are reported in ppm relative to 85% aqueous H 3 PO 4 and CFCl 3 , respectively. Solution phase magnetic measurements were performed by the method of Evans. 46 Optical spectroscopy measurements were taken on a Cary 50 UV-Vis spectrophotometer using a 1-cm two-window quartz cell. Electrochemical measurements were carried out in a glovebox under a dinitrogen atmosphere in a one compartment cell using a CH Instruments 600B electrochemical analyzer. A glassy carbon electrode was used as the working electrode and platinum wire was used as the auxiliary electrode. The reference electrode was Ag/AgNO 3 in THF. The ferrocene couple Fc+/Fc was used as an internal reference. Solutions (THF) of electrolyte (0.2 M tetra-n-butylammonium hexafluorophosphate) and analyte were also prepared under an inert atmosphere.

X-ray Crystallography. XRD studies were carried out at the Beckman Institute Crystallography Facility on a Bruker Kappa Apex II diffractometer (Mo Kα radiation). Structures were solved using SHELXS and refined against F 2 on all data by full-matrix least squares with SHELXL. 47 The crystals were mounted on a wire loop. Methyl group hydrogen atoms not involved in disorder were placed at calculated positions starting from the point of maximum electron density. All other hydrogen atoms, except where otherwise noted, were placed at geometrically calculated positions and refined using a riding model. The isotropic displacement parameters of the hydrogen atoms were fixed at 1.2 (1.5 for methyl groups) times the U eq of the atoms to which they are bonded.

Computations. A single-point calculation and Natural Bond Orbital (NBO) analysis was carried out on [(CP iPr 3 )FeN 2 ][K(Et 2 O) 3 ] (12) using the crystallographically determined atomic coordinates at the B3LYP/6-31++G(d,p) level of theory using the Gaussion03 suite of programs. 48 NBO analysis located a polarized a interaction between Fe and the C-atom anchor (C01).

10-phenyl-10H-dibenzo[b,e]iodininium Bromide (2). The procedure for the generation of 2 and 4 (below) was adapted from a reported method for the generation of diaryliodonium salts. 29 3-chloroperoxybenzoic acid (9.0 g, ˜70% by mass, ˜0.037 mol) was dissolved in dichloromethane (150 mL) and cooled to 0° C. 2-iodotriphenylmethane (11.7 g, 0.0316 mol) was added as a solid in portions over the course of 10 minutes, during which time there was no observable change to the reaction mixture. This mixture was stirred at 0° C. for 10 minutes and then neat trifluoromethanesulfonic acid (8.74 mL, 0.0990 mol) was added via syringe over the course of 5 minutes. The reaction mixture turned dark brown. After an additional 20 minutes, the reaction mixture was allowed to warm to room temperature and stirred for one hour, and then the solvent was removed in vacuo. The solid material was suspended in 200 mL of diethyl ether and 200 mL of water, and then solid sodium bromide (14 g, 0.136 mol) was added and the mixture was shaken vigorously for 5 minutes, during which time a fine off-white precipitate developed. The precipitate was collected atop a sintered glass frit and washed copiously with water and diethyl ether (14.2 g, 0.0316 mol, quant). 1 H NMR ((CD 3 ) 2 S═O, 300 MHz, 298 K, δ): 8.27 (dd, J=8 Hz, 1 Hz, 2H), 7.68 (td, J=8 Hz, 1 Hz, 2H), 7.46 (td, J=8 Hz, 1 Hz), 7.27 (m, 3H), 6.78 (dm, J=8 Hz, 2H), 6.09 (s, 1H) ppm. 13 C NMR ((CD 3 ) 2 S═O, 75.4 MHz, 298 K, δ): 140.3 (s), 138.3 (s), 135.0 (s), 132.7 (s), 131.7 (s), 129.6 (s), 128.9 (s), 127.9 (s), 127.4 (s), 117.4 (s), 57.7 (s) ppm. ESI-MS (positive ion, amu): Calc. 370.0; Found 370.0.

2-bromo-2′-iodotriphenylmethane (3). 10-phenyl-10H-dibenzo[b,e]iodininium bromide (16.11 g, 0.0358 mol) was suspended in dry, degassed acetonitrile (250 mL), and solid tetrabutylammonium bromide (25 g, 0.078 mol) and copper(I) bromide (8 g, 0.06 mol) were added. The mixture was heated to a vigorous reflux and stirred at reflux for five days. The dark brown reaction mixture was then concentrated to dryness in vacuo, extracted with toluene, and filtered through a silica plug. The pale yellow filtrate was concentrated to dryness and the resulting material was recrystallized from methanol to give the desired product as an off-white powder which was collected atop a sintered glass frit and washed with cold methanol (12.7 g, 0.0282 mol, 79%). 1 H NMR (CDCl 3 , 300 MHz, 298 K, δ): 7.90 (dd, J=8 Hz, 1 Hz, 1H), 7.60 (dd, J=8 Hz, 1 Hz, 1H), 7.34-7.18 (m, 5H), 7.13 (td, J=8 Hz, 1 Hz, 1H), 7.03 (dd, J=8 Hz, 1 Hz, 2H), 6.95 (td, J=8 Hz, 1 Hz, 1H), 6.79 (dd, J=8 Hz, 1 Hz, 2H), 6.02 (s, 1H) ppm. 13 C NMR (CDCl 3 , 75.4 MHz, 298 K, δ): 145.2 (s), 142.2 (s), 141.1 (s), 140.1 (s), 133.1 (s), 131.2 (s), 130.7 (s), 130.0 (s), 128.5 (s), 128.3 (s), 128.2 (s), 128.0 (s), 127.2 (s), 126.7 (s), 126.3 (s), 102.9 (s), 60.8 (s) ppm. MS (amu): Calc. 449.9, 447.9; Found 449.9, 447.9.

10-(2-bromophenyl)-10H-dibenzo[b,e]iodininium iodide (4). 3-chloroperoxybenzoic acid (5 g, ˜70% by mass, ˜0.0203 mol) was dissolved in dichloromethane (200 mL) and cooled to 0° C. 2-bromo-2′-iodotriphenylmethane (8.2 g, 0.0182 mol) was added as a solid in portions over the course of 10 minutes, during which time there was no observable change in the reaction mixture. This mixture was stirred at 0° C. for 10 minutes and then neat trifluoromethanesulfonic acid (5.04 mL, 0.0571 mol) was added via syringe over the course of 5 minutes. The reaction mixture turned dark brown. After an additional 30 minutes, the reaction mixture was allowed to warm to room temperature and stirred for 30 minutes, and then the solvent was removed in vacuo. The solid material was suspended in 200 mL of diethyl ether and 200 mL of water, and then solid potassium iodide (15 g, 0.090 mol) was added and the mixture was shaken vigorously for 5 minutes, during which time a fine yellow precipitate developed. The precipitate was collected atop a sintered glass frit and washed copiously with water and diethyl ether (9.95 g, 0.0173 mol, 95%). 1 H NMR ((CD 3 ) 2 S═O, 300 MHz, 298 K, δ): 8.20 (dd, J=8 Hz, 1 Hz, 2H), 7.83 (dd, J=8 Hz, 1 Hz, 2H), 7.72 (dd, J=8 Hz, 1 Hz, 1H), 7.60 (td, J=8 Hz, 1 Hz, 2H), 7.47-7.39 (m, 3H), 7.33 (td, J=8 Hz, 1 Hz, 1H), 7.23 (dd, J=8 Hz, 1 Hz, 1H), 6.02 (s, 1H) ppm. 13 C NMR ((CD 3 ) 2 S═O, 75.4 MHz, 298 K, δ): 138.9 (s), 135.4 (s), 135.1 (s), 135.0 (s), 133.4 (s), 132.8 (s), 131.7 (s), 130.7 (s), 130.0 (s), 128.0 (s), 117.2 (s), 110.0 (s), 58.8 (s) ppm. ESI-MS (positive ion, amu): Calc. 446.9, 448.9; Found 446.9, 448.9.

2-bromo-2′,2″-diiodotriphenylmethane (5): Solid 10-(2-bromophenyl)-10H-dibenzo[b,e]iodininium iodide (4.54 g, 7.88 mmol) was sealed inside a Schlenk tube under N 2 and heated to 200° C. for 15 minutes, and then cooled to room temperature. The resulting dark violet residue was taken up in dichloromethane (50 mL) and washed with saturated aqueous sodium thiosulfate (50 mL) and then water (30 mL) and saturated aqueous sodium chloride (30 mL), then dried over magnesium sulfate, filtered, and concentrated to dryness in vacuo. The resulting off-white residue was recrystallized from methanol to give the desired product as a fine white powder, which was collected atop a sintered glass frit and washed with cold methanol (3.4 g, 5.90 mmol, 75%). 1 H NMR (CDCl 3 , 300 MHz, 298 K, δ): 7.93 (d, J=8 Hz, 2H), 7.64 (d, J=8 Hz, 1H), 7.30-7.16 (m, 4H), 7.00 (t, J=8 Hz, 2H), 6.72 (d, J=8 Hz, 3H), 6.04 (s, 1H) ppm. 13 C NMR (CDCl 3 , 75.4 MHz, 298 K, δ): 144.1 (s), 141.1 (s), 140.2 (s), 133.3 (s), 131.1 (s), 130.7 (s), 128.6 (s), 128.5 (s), 127.3 (s), 126.7 (s), 103.6 (s), 65.4 (s) ppm. MS (amu): Calc. 573.8, 575.8; Found 446.9, 448.9 ([M-I]+), 368.1 ([M-I-Br]+), 320.1, 322.1 ([M-2I]+).

Tris(2-(diisopropylphosphino)phenyl)methane (“(C iPr P 3 )H”) (1): 2-bromo-2′,2″-diiodotriphenylmethane (2.00 g, 3.48 mmol) was dissolved in diethyl ether (100 mL) and cooled to −78° C. while stirring. Solid t-butyllithium (1.36 g, 21.23 mmol) was added in portions over the course of 10 minutes and the reaction mixture was stirred at low temperature for 3 hours. Then chlorodiisopropylphosphine (1.96 g, 12.8 mmol) was dissolved in 10 mL of diethyl ether and added to the reaction mixture. The reaction mixture was allowed to warm slowly to room temperature overnight, resulting in the precipitation of a fine white solid. The reaction mixture was filtered through silica and the pale yellow-orange filtrate was concentrated to a sticky yellow solid which was triturated with acetonitrile to give an off-white powder. The solid was washed copiously with acetonitrile and then dried under vacuum, giving 1.4 g (2.36 mmol, 68%) of the desired product. 1 H NMR (C 6 D 6 , 300 MHz, 298 K, δ): 8.15 (q, J=6 Hz, 1H), 7.44 (d, J=7 Hz, 3H), 7.06 (td, J=7 Hz, 2 Hz, 3H), 7.00-6.93 (m, 6H), 2.27 (septet of doublets, J=4 Hz, 7 Hz, 3H), 1.73 (septet of doublets, J=3 Hz, 7 Hz, 3H), 1.40 (dd, J=7 Hz, 13 Hz, 9H), 1.32 (dd, J=7 Hz, 12 Hz, 9H), 0.88 (dd, J=7 Hz, 13 Hz, 9H), 0.44 (dd, J=7 Hz, 12 Hz, 9H) ppm. 13 C NMR (C 6 D 6 , 75.4 MHz, 298 K, δ): 159.0 (d, J=29 Hz), 144.8 (d, J=17 Hz), 140.0 (s), 139.3 (s), 132.4 (s), 59.1 (m), 32.7 (m), 30.0 (m), 29.4 (s), 27.3 (m), 21.0 (s) ppm. 31 P NMR (C 6 D 6 , 121.4 MHz, 298 K, δ): −9.1 ppm. Anal. Calcd. for C 37 H 55 P 3 : C, 74.97; H, 9.35. Found: C, 74.73; H, 9.49.

{(CP iPr 3 )H}FeI 2 (6): (CP iPr 3 )H (500 mg, 0.843 mmol) was added to FeI 2 (350 mg, 1.13 mmol) in 15 mL of toluene and stirred at 60° C. for 2 hours, at which point the reaction mixture was filtered through Celite and the yellow filtrate was concentrated to give a yellow powder (761 mg, 0.843 mmol, quant). Crystals suitable for X-ray diffraction were grown by layering of pentane over a saturated toluene solution. 1 H NMR (C 6 D 6 , 300 MHz, 298 K, δ): 179.69, 26.00, 18.60, 14.92, 14.28, 13.62, 12.74, 9.96, 9.00, 8.29, 6.76, 6.16, 5.72, 5.48, 4.97, 4.28, 3.78, 0.30, 0.13, −0.48, −0.91, −2.02, −3.68, −5.09, −9.45 ppm. μ eff (C 6 D 6 , Evans' method, 298 K): 4.85 μ B .

(CP iPr 3 )Fe(N 2 )H (9): (CP iPr 3 )HFeI 2 (370 mg, 0.410 mmol) was suspended in benzene (10 mL) and stirred vigorously over an excess of 0.7% sodium/mercury amalgam (25 mg Na, 1.1 mmol) for two hours. The initially yellow suspension turned a deep brick red color during this time due to the formation of {(CP iPr 3 )H}FeI (7). The reaction mixture was filtered through Celite and concentrated to dryness in vacuo. The deep red residue was then suspended in diethyl ether (15 mL) at −78° C. and 3 mL of dimethoxyethane was added; this solution was vigorously stirred over excess sodium mirror for 4 hours at −78° C., during which time the color lightened to orange. The reaction mixture was then filtered through Celite and concentrated to dryness. The residue was extracted into pentane and again filtered through Celite, giving a lighter yellow-orange filtrate which was concentrated to dryness again. This residue could be recrystallized from diethyl ether by slow evaporation to give yellow crystalline solids. These solids were washed with hexamethyldisiloxane and minimal cold diethyl ether, and then dried in vacuo to give 155 mg (0.229 mmol, 56%) of the desired product. Crystals suitable for X-ray diffraction were grown by evaporation of a concentrated pentane solution into hexamethyldisiloxane. 1 H NMR (C 6 D 6 , 300 MHz, 298 K, δ): 7.57 (t, J=6 Hz, 1H), 7.34 (m, 1H), 7.08 (m, 2H), 6.96 (m, 2H), 6.83-6.75 (m, 4H), 6.65 (m, 1H), 6.50 (m, 1H), 2.94 (septet, J=8 Hz, 1H), 2.75 (m, 2H), 2.36 (septet, J=6 Hz, 1H), 2.05 (septet, J=7 Hz, 1H), 1.75-1.17 (m, 25H), 1.02 (dd, J=7 Hz, 11 Hz, 3H), 0.65 (dd, J=7 Hz, 15 Hz, 3H), 0.56 (dd, J=7 Hz, 10 Hz, 3H), 0.27 (dd, J=8 Hz, 13 Hz, 3H), −10.2 (ddd, J=38 Hz, 53 Hz, 50 Hz) ppm. 31 P NMR (C 6 D 6 , 121.4 MHz, 298 K, δ): 90.1 (dt, J=100 Hz, 17 Hz, 1P), 67.0 (m, 1P), 63.4 (dt, J=100 Hz, 17 Hz, 1P) ppm. IR (thin film; cm −1 ): 2046 (N—N), 1920 (Fe—H). Anal. Calcd. for C 37 H 55 FeP 3 N 2 : C, 65.68; H, 8.19; N, 4.14. Found: C, 65.91; H, 7.89; N, 3.94.

{(CP iPr 3 )H}FeBr (8): {(CP iPr 3 )H}FeBr 2 (5.0 mg, 0.0070 mmol, generated by treating CP 3 H with anhydrous FeBr 2 in toluene) was dissolved in toluene, cooled to −78° C., and treated with isopropyl magnesium chloride (3.5 μL, 2.0M in Et 2 O). The reaction mixture rapidly turned dark brick-red. It was stirred at low temperature for one hour and then allowed to warm to room temperature for thirty minutes before being filtered and concentrated. The dark red powder was not purified, but was analyzed by NMR in C 6 D 6 , and X-ray quality crystals were grown by layering pentane over a filtered benzene solution.

(CP iPr 3 )FeCl (10): (CP iPr 3 )Fe(N 2 )H (61 mg, 0.0901 mmol) was dissolved in diethyl ether (8 mL) and cooled to −78° C. HCl in diethyl ether (1.0 M, 108 μL, 0.108 mmol) was added to the solution in one portion. The reaction mixture was stirred at low temperature for one hour and then warmed to room temperature and stirred overnight. The color darkened to deep red-orange, and the reaction mixture was filtered through Celite and concentrated to dryness. The red residue was recrystallized by evaporation of a pentane solution into hexamethyldisiloxane and the resulting dark red crystals were washed sparingly with cold pentane and dried in vacuo, giving 46 mg (0.0673 mmol, 75%) of (CP iPr 3 )FeCl. Crystals suitable for X-ray diffraction were grown by evaporation of a concentrated pentane solution into hexamethyldisiloxane. 1 H NMR (C 6 D 6 , 300 MHz, 298 K, δ): 179.93, 26.47, 23.05, 17.44, 17.22, 15.03, 11.66, 1.52, −10.27, −13.36, −16.82 ppm. μ eff (C 6 D 6 , Evans' method, 298 K): 4.92 μ B . Anal. Calcd. for C 37 H 54 FeP 3 Cl: C, 65.06; H, 7.97. Found: C, 64.96; H, 8.01.

(CP iPr 3 )FeN 2 (11): (CP iPr 3 )FeCl (82 mg, 0.120 mmol) was dissolved in THF (2 mL) and stirred over sodium mirror for 20 minutes, or until NMR analysis showed complete consumption of the starting material, and then filtered and concentrated. The residue was extracted with pentane and filtered through Celite, and concentrated to a brownish-orange residue which was recrystallized by evaporation of a pentane solution into hexamethyldisiloxane. The dark brown-orange crystals were washed with hexamethyldisiloxane and cold pentane and dried in vacuo to give 39 mg (0.0581 mmol, 48%) of (CP iPr 3 )FeN 2 . Crystals suitable for X-ray diffraction were grown by evaporation of a concentrated pentane solution into hexamethyldisiloxane. 1 H NMR (C 6 D 6 , 300 MHz, 298 K, δ): 19.3 (very broad), 10.4, 6.8, 3.0, 2.0, 0.6, −1.4 ppm. μ eff (C 6 D 6 , Evans' method, 298 K): 1.75 μ B . IR (thin film; cm −1 ): 1992 (N—N). Anal. Calcd. for C 37 H 54 FeP 3 N 2 : C, 65.78; H, 8.06; N, 4.15. Found: C, 66.03; H, 8.01; N, 3.86.

[(CP iPr 3 )FeN 2 ][K(Et 2 O) 0.5 ] (12[K(Et 2 O) 0.5 ]): (CP iPr 3 )FeCl (40 mg, 0.0586 mmol) was dissolved in diethyl ether (5 mL) at room temperature and an excess of potassium graphite (KC 8 , 25 mg) was added. The reaction mixture was stirred for 10 minutes and then filtered through Celite. The dark brown solution was concentrated to about 2 mL and then pentane was layered over the ether solution and it was allowed to stand overnight during which time dark bluish-brown crystals formed. The supernatant was decanted and the crystals were washed thoroughly with pentane and thoroughly dried under vacuum, giving 26 mg of the desired product (0.0277 mmol, 47%). NMR analysis indicates the presence of 0.5 ether solvent molecules per anion. Crystals suitable for X-ray diffraction were grown by vapor diffusion of pentane into a diethyl ether solution; in these crystals the potassium cation is solvated by three diethyl ether molecules. 1 H NMR (d 8 -THF, 300 MHz, 298 K, δ): 7.04 (s, 3H), 6.67 (s, 3H), 6.47 (s, 6H), 3.38 (q, J=7 Hz, 2H, diethyl ether (CH 3 CH 2 ) 2 O), 2.99 (br s, 3H), 2.14 (br s, 3H), 1.42 (d, J=6 Hz, 9H), 1.36 (d, J=5 Hz, 9H), 1.12 (t, J=7 Hz, 3H, diethyl ether (CH 3 CH 2 ) 2 O), 1.01 (d, J=5 Hz, 9H), 0.12 (d, 9H) ppm. 31 P NMR (5:1 C 6 D 6 /d 8 -THF, 121.4 MHz, 298 K, δ): 68.1 ppm. IR (thin film deposited from Et 2 O; cm −1 ): 1870 (N—N).

[(CP iPr 3 )FeN 2 ][K(12-c-4) 2 ] (12[K(12-c-4) 2 ]). A sample of 12 (15 mg, 0.020 mmol) was dissolved in diethyl ether (1 mL) and 12-crown-4 (8.8 mg, 0.050 mmol) was added as a solution in diethyl ether (1 mL). The resulting solution was layered with pentane and allowed to stand overnight, resulting in the crystallization of 12[K(12-crown-4) 2 ] as a very dark blue solid. The crystals were washed with pentane and dried under vacuum, giving 10 mg of material (53% yield). 1 H NMR (d 8 -THF, 300 MHz, 298 K, δ) 6.86 (br s, 6H), 6.47 (s, 6H), 3.62 (s, 36H, 12-crown-4), 1.43 (s, 9H), 1.30 (s, 9H), 0.91 (s, 9H), 0.16 (s, 9H) ppm. 31 P (C 6 D 6 , 121.4 MHz, 298 K, δ): 66 ppm. IR (thin film; cm −1 ) 1905 (N—N).

[(CP iPr 3 )FeN 2 ][B(3,5-(CF 3 ) 2 —C 6 H 3 ) 4 ] (13): (C iPr P 3 )FeN 2 (7.3 mg) was dissolved in diethyl ether (1 mL) and a solution of [Fe(C 5 Me 5 ) 2 ][B(3,5-(CF 3 ) 2 —C 6 H 3 ) 4 ] in diethyl ether (1 mL) was added dropwise while stirring at room temperature. The reaction mixture was then concentrated to give an orange solid which was washed with benzene and then dried in vacuo. Crystals suitable for X-ray diffraction were grown by slow evaporation of a diethyl ether solution into hexamethyldisiloxane. 1 H NMR (4:1 C 6 D 6 /THF-d 8 under N 2 , 300 MHz, 298 K, δ): 16.65, 14.48, 8.15, 7.60, 2.71 ppm. (Note: the exact position of the paramagnetically shifted NMR peaks varies with the composition of the solvent due to the likely exchange of the N 2 ligand with THF). μ eff (d 8 -THF, Evans' method, 298 K): 4.3 μ B . IR (thin film; cm −1 ): 2128 (N—N). Satisfactory elemental analysis could not be obtained due to the lability of the coordinated N 2 ligand.

(CP iPr 3 )FeN 2 SiMe 3 (14): [(CP iPr 3 )FeN 2 ][K(Et 2 O) 0.5 ] (35 mg, 0.0465 mmol) was dissolved in diethyl ether (2 mL) and cooled to −78° C. Trimethylsilyl chloride (6 μL, 0.0473 mmol) was dissolved in diethyl ether (1 mL) and added dropwise to the stirring reaction mixture. The reaction was stirred at low temperature for one hour and then warmed to room temperature for one hour, concentrated to dryness, taken up in pentane, filtered through Celite, and concentrated. The red-orange residue was recrystallized by slow evaporation of a pentane solution into hexamethyldisiloxane, and the resulting red solids were washed with cold hexamethyldisiloxane and dried in vacuo to give 21 mg (0.0280 mmol, 60%) of solid material, which was contaminated with a small amount of CP 3 FeN 2 (11) which we were unable to remove by repeated recrystallization. 14 decomposes slowly to 11 over time. 1 H NMR (C 6 D 6 , 300 MHz, 298 K, δ) 7.33 (br m, 3H), 6.80 (t, J=4 Hz, 6H), 6.63 (m, 3H), 2.67 (septet, J=7 Hz, 3H), 1.97 (septet, J=7 Hz, 3H), 1.45 (m, 18H), 0.96 (q, J=7 Hz, 9H), 0.72 (q, J=7 Hz, 9H), 0.12 (s, 3H) ppm. 31 P (C 6 D 6 , 121.4 MHz, 298 K, δ): 80.1 ppm. IR (thin film; cm −1 ) 1736 (N—N).

Ammonia Quantification. A Schlenk tube was charged with HCl (4 mL of a 1.0 M solution in Et 2 O, 4 mmol). Reaction mixtures were vacuum transferred into this collection flask. Residual solid in the reaction vessel was treated with a solution of [Na][O-t-Bu] (40 mg, 0.4 mmol) in 1,2-dimethoxyethane (1 mL) and sealed. The resulting suspension was allowed to stir for 10 minutes before all volatiles were again vacuum transferred into the collection flask. After completion of the vacuum transfer, the flask was sealed and warmed to room temperature. Solvent was removed in vacuo and the remaining residue was dissolved in H 2 O (1 mL). An aliquot of this solution (20 μL) was then analyzed for the presence of NH 3 (trapped as [NH 4 ][Cl]) via the indophenol method. 38 Quantification was performed with UV-Vis spectroscopy by analyzing absorbance at 635 nm.

Standard catalytic procedure with [(CP iPr 3 )FeN 2 ][K(Et 2 O) 0.5 ] (12): [(CP iPr 3 )FeN 2 ][K(Et 2 O) 0.5 ] (1.9 mg, 0.0025 mmol) was dissolved in Et 2 O (0.5 mL) in a small Schlenk tube equipped with a stir bar. This solution was cooled to −78° C. in a cold well inside of the glove box. A suspension of KC 8 (14 mg, 0.100 mmol) in Et 2 O (0.75 mL) was cooled to −78° C. and added to the reaction mixture with stirring. After five minutes, a similarly cooled solution of HBAr F 4 .2 Et 2 O (93 mg, 0.092 mmol) in Et 2 O (1.0 mL) was added to the suspension in one portion with rapid stirring. Any remaining acid was dissolved in cold Et 2 O (0.25 mL) and added subsequently, and the Schlenk tube was sealed. The reaction was allowed to stir for 60 minutes at −78° C. before being warmed to room temperature and stirred for 15 minutes.

1. Smil, V. Enriching the Earth (MIT Press, Cambridge, 2001).

2. (a) Burgess, B. K.; Lowe, D. J. Chem. Rev. 1996, 96, 2983-3011. (b) Eady, R. R. Chem. Rev. 1996, 96, 3013-3030.

3. Peters, J. C.; Mehn, M. P. in Activation of Small Molecules: Organometallic and Bioinorganic Perspectives ed. Tolman, W. B.) 81-119 (Wiley-VCH, 2006).

4. Hazari, N. Chem. Soc. Rev. 2010, 39, 4044.

5. MacLeod, K. C.; Holland, P. L. Nature Chem. 2013, 5, 559.

6. Crossland, J. L.; Tyler, D. R. Coord. Chem. Rev. 2010, 254, 1883.

7. Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76.

8. Arashiba, K.; Miyake, Y.; Nishibayashi, Y. Nature Chem. 2011, 3, 120.

9. Weare, W. W.; Dai, X.; Byrnes, M.; Chin, J.-M.; Schrock, R. R.; Muller, P. Proc. Natl. Acad. Sci. 2006, 103, 17099.

10. (a) Saouma, C. T.; Lu, C. C.; Peters, J. C. Inorg. Chem. 2012, 51, 10043. (b) Saouma, C. T.; Kinney, R. A.; Hoffman, B. M.; Peters, J. C. Angew. Chem. Int. Ed. 2011, 50, 1. (c) Saouma, C. T.; Muller, P.; Peters, J. C. J. Am. Chem. Soc. 2009, 131, 10358.

11. Smith, J. M. Lachiotte, R. J.; Pittard, K. A.; Cundari, T. R.; Lukat-Rodgers, G.; Holland, P. L. J. Am. Chem. Soc. 2001, 123, 9222.

12. Field, L. D.; Li, H.; Magill, A. M. Inorg. Chem. 2001, 48, 5.

13. Li, Y.; Li, Y.; Wang, B.; Luo, Y.; Yang, D.; Tong, P.; Zhao, J.; Luo, L.; Zhou, Y.; Chen, S.; Cheng, F.; Qu, J. Nature Chem. 2013, 5, 320.

14. Rodriguez, M. M.; Bill, E.; Brennessel, W. W.; Holland, P. L. Science 2011, 334, 780.

15. Moret, M. E.; Peters, J. C.; J. Am. Chem. Soc, 2011, 133, 18118.

16. Yuki, M.; Tanaka, H.; Sasaki, K.; Miyake, Y.; Yoshizawa, K.; Nishibayashi, Y. Nature Comm. 2012, 3, 1254.

17. Lee, Y.; Mankad, N. P., Peters, J. C. Nature Chem. 2012, 2, 558.

18. (a) Leigh, G. J.; Jimenez-Tenorio, M. J. Am. Chem. Soc. 1991, 113, 5862. (b) Hall, D. A.; Leigh, G. J. J. Chem. Soc. Dalton Trans. 1996, 3539. (c) Gilbertson, J. D.; Szymczak, N. K.; Tyler, D. R. J. Am. Chem. Soc. 2005, 127, 10184. (d) Yamamoto, A.; Miura, Y.; Ito, T.; Chen, H. L.; Iri, K.; Ozawa, F.; Miki, K.; Sei, T.; Tanaka, N.; Kasai, N. Organometallics 1993, 2, 1429. (e) George, T. A.; Rose, D. J.; Chang, Y.; Chen, Q.; Zubieta, J. Inorg. Chem. 1995, 34, 1295. (f) Borodko, Y. G.; Broitman, M. O.; Kachapina, L. M.; Shilov, A. E.; Ukhin, L. Y. J. Chem. Soc. D, 1971, 1185.

19. Einsle, O.; Tezcan, A.; Andrade, S. L. A.; Schmid, B.; Yoshida, M.; Howard, J. B.; Reese, D. C. Science 2002, 297, 1696.

20. (a) Spatzal, T.; Aksoyoglu, M.; Zhang, L.; Andrade, S. L. A.; Schleicher, E.; Weber, S.; Rees, D. C.; Einsle, O. Science 2011, 334, 940. (b) Lancaster, K. M.; Roemelt, M.; Ettenhuber, P.; Hu, Y.; Ribbe, M. W.; Neese, F.; Bergmann, U.; DeBeer, S. Science 2011, 334, 974-7. (c) Lancaster, K. M.; Hu, Y.; Bergmann, U.; Ribbe, M. W.; DeBeer, S. J. Am. Chem. Soc. 2013, 135, 610-2. (d) Wiig, J. A.; Hu, Y.; Lee, C. C.; Ribbe, M. W. Science, 2012, 337, 1672-1675.

21. Anderson, J. S.; Rittle, J.; Peters, J. C. Nature 2013, 501, 84.

22. Rittle, J.; Peters, J. C. Proc. Natl. Acad. Sci. U.S.A . [Online early access]. DOI: 10.1073/pnas.1310153110. Published Online: Sep. 16, 2013.

23. Macbeth, C. E.; Harkins, S. B.; Peters, J. C. Can. J. Chem. 2005, 83, 332.

24. Mankad, N. P.; Whited, M. T.; Peters, J. C. Angew. Chem. Int. Ed. 2007, 46, 5768.

25. Moret, M.-E.; Peters, J. C. Angew. Chem. Int. Ed. 2011, 50, 2063.

26. Bontemps, S.; Bouhadir, G.; Dyer, P. W.; Miqueu, K.; Bourissou, D. Inorg. Chem. 2007, 46, 5149.

27. Lesueur, W.; Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Inorg. Chem. 1997, 36, 3354.

28. Bickelhaupt, F.; Jongsma, C.; de Koe, P.; Lourens, R.; Mast, N. R.; van Mourik, G. L.; Vermeer, H.; Weustink, R. J. M. Tetrahedron 1976, 32, 1921.

29. Bielawski, M.; Olofsson, B. Chem. Commun. 2007, 2521.

30. Ciclosi, M.; Lloret, J.; Estevan, F.; Lahuerta, P.; Sanau, M.; Perez-Prieto, J. Angew. Chem. Int. Ed. 2006, 45, 6741.

31. Schlosser, M. Pure Appl. Chem. 1988, 60, 1627.

32. Hoffmann, R.; Bissel, R.; Farnum, D. G. J. Phys. Chem. 1969, 73, 1789.

33. Lee, Y.; Kinney, R. A.; Hoffman, B. M.; Peters, J. C. J. Am. Chem. Soc. 2011, 133, 16366.

34. Field, L. D.; Guest, R. W.; Vuong, K. Q.; Dalgarno, S. J. Jensen, P. Inorg. Chem. 2009, 48, 2246.

35. Anderson, J. S.; Moret, M.-E.; Peters, J. C. J. Am. Chem. Soc. 2013, 135, 534.

36. Weinhold, F.; Landis, C. (2005) Valency and Bonding: A Natural Bond Orbital Donor - Acceptor Perspective (Cambridge, Univ. Press, Cambridge).

37. Hills, A.; Hughes, D. L.; Jimenez-Tenorio, M.; Leigh, G. F.; Rowley, A. T. J. Chem. Soc. Dalton Trans. 1993, 25, 3041.

38. Weatherburn, M. W. Anal. Chem. 1967, 39, 971.

39. (a) Yang, Z.-Y.; Khadka, N.; Lukoyanov, D.; Hoffman, B. M.; Dean, D. R.; Seefeldt, L. C. Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 16327. (b) Simpson, F. B.; Burris, R. H. Science 1984, 224, 1095.

40. (a) Tard, C.; Liu, X.; Ibrahim, S. K.; Maurizio, B.; DeGioia, L.; Davis, S. C.; Yang, X.; Wang, L.-S.; Sawers, G.; Pickett, C. J. Nature 2005, 433, 610. (b) Gloaguen, F.; Rauchfuss, T. B. Chem. Soc. Rev. 2009, 38, 100. (c) Darensbourg, M. Y.; Lyon, E. J.; Smee, J. J. Coord. Chem. Rev. 2000, 206-207, 533. (d) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C. Science 1998, 282, 1853. (e) Volbeda, A.; Charon, M.-H.; Piras, C.; Hatchikian, E. C.; Frey, M.; Fontecilla-Camps, J. C. Nature 1995, 373, 580. (f) Vincent, K. A.; Parkin, A.; Armonstrong, F. A. Chem. Rev. 2007, 107, 4366.

41. (a) Chatt, J.; Dilworth, J. R.; Richards, R. L. Chem. Rev. 1978, 78, 589. (b) Seefeldt, L.; Hoffman, B. M.; Dean, D. R. Annu. Rev. Biochem. 2009, 78, 701. (c) Lukoyanov, D.; Dikanov, S. A.; Yang, Z.-Y.; Barney, B. M.; Samoilova, R. I.; Narasimhulu, K. V.; Dean, D. R.; Seefeldt, L. C.; Hoffman, B. M. J. Am. Chem. Soc. 2011, 133, 11655.

42. Brookhart, M.; Grant, B.; Volpe, A. F. Jr. Organometallics, 1992, 11, 3920.

43. Wietz, I. S.; Rabinovitz, M. J. J. Chem. Soc. Perkin Trans., 1993, 1, 117.

44. Chavez, I.; Alvarez-Carena, A.; Molins, E.; Roig, A.; Maniukiewicz, W.; Arancibia, A.; Arancibia, V.; Brand, H.; Manriquez, J. M. J. Organomet. Chem. 2000, 601, 126.

45. Job, R.; Earl, R. Inorg. Nuc. Chem. Lett. 1979, 15, 21.

46. Evans, D. F. J. Chem. Soc. 1959, 2003.

47. Sheldrick, G. M. Acta Cryst. A. 2008, 82, 169.

48. Gaussian 03, Revision C.02, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; lyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; and Pople, J. A.; Gaussian, Inc., Wallingford Conn., 2004.

Synthesis of o-nitrotriphenylmethane: (Note: The synthesis of this species, a precursor to o-iodotriphenylmethane, has been reported; 1 here we report the synthesis again in order to detail slight modifications to the workup and purification procedures that facilitate its synthesis. The syntheses of o-aminotriphenylmethane and o-iodotriphenylmethane have not been substantially modified from the reported procedures.) Aluminum chloride (30.36 g, 0.228 mol) was suspended in 100 mL of benzene and o-nitrobenzaldehyde (15 g, 0.099 mol) was added as a solid in portions over 15 minutes while stirring at room temperature. The reaction mixture was heated to reflux for 6 hours, then cooled to room temperature and poured over ice (300 mL). The mixture was diluted with another 100 mL of benzene and 100 mL of water. The aqueous layer was removed and washed twice with benzene (100 mL); the dark brown organic washings were then combined and washed repeatedly with concentrated H 2 SO 4 until the color was light yellow-orange. The organic layer was then washed with water (100 mL) and brine (2×50 mL), dried over magnesium sulfate, filtered, and concentrated. The off-white oily residue was recrystallized from methanol to give a white crystalline solid which was collected atop a glass frit and washed with cold methanol (15.6 g, 54%). 1 H NMR (CDCl 3 , 300 MHz, 298 K, δ): 7.89 (d, J=8 Hz, 1H), 7.49 (t, J=8 Hz, 1H), 7.39 (t, J=8 Hz, 1H), 7.34-7.23 (m, 6H), 7.14-7.08 (m, 4H) ppm. 13 C NMR (CDCl 3 , 75.4 MHz, 298 K, δ): 149.77 (s), 141.95 (s), 138.13 (s), 132.41 (s), 132.04 (s), 129.51 (s), 128.54 (s), 127.49 (s), 126.85 (s), 124.76 (s) ppm.

Catalytic Production of NH 3 using [(CP iPr 3 )FeN 2 ][K(Et 2 O) 0.5 ]

Standard catalytic procedure with [(CP iPr 3 )FeN 2 ][K(Et 2 O) 0.5 ] (12): [(CP iPr 3 )FeN 2 ][K(Et 2 O) 0.5 ] (1.9 mg, 0.0025 mmol) was suspended in Et 2 O (0.5 mL) in a small Schlenk tube equipped with a stir bar. This solution was cooled to −78° C. in a cold well inside of the glove box. A suspension of KC 8 (14 mg, 0.100 mmol) in Et 2 O (0.75 mL) was cooled to −78° C. and added to the reaction mixture with stirring. After five minutes, a similarly cooled solution of HBAr F 4 .2 Et 2 O (93 mg, 0.092 mmol) in Et 2 O (1.0 mL) was added to the suspension in one portion with rapid stirring. Any remaining acid was dissolved in cold Et 2 O (0.25 mL) and added subsequently, and the Schlenk tube was sealed. The reaction was allowed to stir for 60 minutes at −78° C. before being warmed to room temperature and stirred for 15 minutes.

Hydrazine was not detected in the catalytic runs using a standard UV-Vis quantification method 2 .

Modified “acid-first” catalytic procedure with [(CP iPr 3 )FeN 2 ][K(Et 2 O) 0.5 ](12): [(CP iPr 3 )FeN 2 ][K(Et 2 O) 0.5 ] (1.9 mg, 0.0025 mmol) was dissolved in Et 2 O (0.5 mL) in a 20 mL scintillation vial equipped with a stir bar. This dark brown solution was vigorously stirred and cooled to −78° C. in a cold well inside of the glove box. A similarly cooled solution of HBAr F 4 .2 Et 2 O (93 mg, 0.092 mmol) in Et 2 O (1.5 mL) was added to the solution in one portion with rapid stirring. Any remaining acid was dissolved in cold Et 2 O (0.25 mL) and added subsequently. The reaction mixture turned light orange upon addition of acid and the resulting solution was allowed to stir for 5 minutes before being transferred into a pre-cooled Schlenk tube equipped with a stirbar. The original reaction vial was washed with cold Et 2 O (0.25 mL) which was subsequently transferred to the Schlenk tube. Solid KC 8 (14 mg, 0.100 mmol) was suspended in cold Et 2 O (0.75 mL) and added dropwise to the rapidly stirred solution in the Schlenk tube which was then tightly sealed. The reaction was allowed to stir for 60 minutes at −78° C. before being warmed to room temperature and stirred for 15 minutes.

Standard Catalytic Procedure with [(TPB)FeN 2 ][Na(12-c-4) 2 ]: [(TPB)FeN 2 ][Na(12-c-4) 2 ] (2.0 mg, 0.002 mmol) was suspended in Et 2 O (0.5 mL) in a small Schlenk tube equipped with a stir bar. This solution was cooled to −78° C. in a cold well inside of the glove box. A suspension of KC 8 (14 mg, 0.100 mmol) in Et 2 O (0.75 mL) was cooled to −78° C. and added to the reaction mixture with stirring. After five minutes, a similarly cooled solution of HBAr F 4 .2 Et 2 O (93 mg, 0.092 mmol) in Et 2 O (1.0 mL) was added to the suspension in one portion with rapid stirring. Any remaining acid was dissolved in cold Et 2 O (0.25 mL) and added subsequently, and the Schlenk tube was sealed. The reaction was allowed to stir for 60 minutes at −78° C. before being warmed to room temperature and stirred for 15 minutes.

Standard catalytic procedure with [(CP iPr 3 )FeN 2 ][K(12-c-4) 2 ] (12[K(12-crown-4) 2 ]): [(CP iPr 3 )FeN 2 ][K(12-c-4) 2 ] (2.0 mg, 0.002 mmol) was suspended in Et 2 O (0.5 mL) in a small Schlenk tube equipped with a stir bar. This solution was cooled to −78° C. in a cold well inside of the glove box. A suspension of KC 8 (14 mg, 0.100 mmol) in Et 2 O (0.75 mL) was cooled to −78° C. and added to the reaction mixture with stirring. After five minutes, a similarly cooled solution of HBAr F 4 .2 Et 2 O (93 mg, 0.092 mmol) in Et 2 O (1.0 mL) was added to the suspension in one portion with rapid stirring. Any remaining acid was dissolved in cold Et 2 O (0.25 mL) and added subsequently, and the Schlenk tube was sealed. The reaction was allowed to stir for 60 minutes at −78° C. before being warmed to room temperature and stirred for 15 minutes.

Standard catalytic procedure with [(SiP iPr 3 )FeN 2 ][Na(12-c-4) 2 ]: [(SiP iPr 3 )FeN 2 ][Na(12-c-4) 2 ] (2.0 mg, 0.002 mmol) was suspended in Et 2 O (0.5 mL) in a small Schlenk tube equipped with a stir bar. This solution was cooled to −78° C. in a cold well inside of the glove box. A suspension of KC 8 (14 mg, 0.100 mmol) in Et 2 O (0.75 mL) was cooled to −78° C. and added to the reaction mixture with stirring. After five minutes, a similarly cooled solution of HBAr F 4 .2 Et 2 O (93 mg, 0.092 mmol) in Et 2 O (1.0 mL) was added to the suspension in one portion with rapid stirring. Any remaining acid was dissolved in cold Et 2 O (0.25 mL) and added subsequently, and the Schlenk tube was sealed. The reaction was allowed to stir for 60 minutes at −78° C. before being warmed to room temperature and stirred for 15 minutes.

Identification of H 2 in a standard catalytic run. A catalytic run was performed with 0.0025 mmol of 12 according to the standard procedure. Prior to the vacuum transfer of volatiles, the solutions inside of the Schlenk tubes were frozen. The ground glass joint of the Schlenk tube was then sealed with a rubber septum and the head space between the Teflon stopcock of the Schlenk tube and the septum was evacuated. This head space was left under static vacuum and the Teflon stopcock of the reaction vessel was opened after which a 10 mL aliquot of the headspace was sampled through the septum via a gas-tight syringe. This sample was then analyzed for hydrogen with an Agilent 7890A gas chromatograph using a thermal conductivity detector. 45% yield of H 2 relative to H + was quantified.

IR spectral analysis of addition of 12 equiv. of KC 8 , followed by 10 equiv of HBAr F 4 .2 Et 2 O to [(CP iPr 3 )FeN 2 ][K(Et 2 O) 0.5 ]. A 20 mL scintillation vial was charged with a stir bar and [(CP iPr 3 )FeN 2 ][K(Et 2 O) 0.5 ] (7.6 mg, 0.0101 mmol). In a separate vial, HBAr F 4 .2 Et 2 O (75 mg, 0.074 mmol) was dissolved in Et 2 O (1 mL). Finally, a third vial was prepared containing a suspension of potassium graphite (12 mg, 0.090 mmol) in Et 2 O (1 mL). All three vials were chilled in the cold well to −78° C. for 10 minutes. The suspension of KC 8 was quickly added to the stirring suspension of [(CP iPr 3 )FeN 2 ][K(Et 2 O) 0.5 ]. After stirring for 5 minutes, HBAr F 4 .2 Et 2 O was added rapidly to the stirring reaction mixture. This solution was capped and stirred at −78° C. for 60 minutes and then brought to r.t. and stirred for 15 minutes. The resulting reaction mixture was concentrated to dryness, taken up in C 6 D 6 , and filtered through Celite, giving an orange solution which was analyzed by IR and NMR. By IR, the major species appeared to be (CP iPr 3 )FeN 2 (11), and the presence of a smaller amount of (CP iPr 3 )Fe(N 2 )(H) (9) was also apparent. By NMR, 11, 9, and (CP iPr 3 )FeCl (10) were detected as well as an additional unidentified diamagnetic species present in small amounts. No uncoordinated (CP iPr 3 )H ligand could be detected.

IR spectral analysis of addition of 40 equiv. of KC 8 , followed by 38 equiv of HBAr F 4 .2 Et 2 O to [(CP iPr 3 )FeN 2 ][K(Et 2 O) 0.5 ]. A 20 mL scintillation vial was charged with a stir bar and [(CP iPr 3 )FeN 2 ][K(Et 2 O) 0.5 ] (8.0 mg, 0.0106 mmol). In a separate vial, HBAr F 4 .2 Et 2 O (392 mg, 0.403 mmol) was dissolved in Et 2 O (1 mL). Finally, a third vial was prepared containing a suspension of potassium graphite (59 mg, 0.424 mmol) in Et 2 O (1 mL). All three vials were chilled in the cold well to −78° C. for 10 minutes. The suspension of KC 8 was quickly added to the stirring suspension of [(CP iPr 3 )FeN 2 ][K(Et 2 O) 0.5 ]. After stirring for 5 minutes, HBAr F 4 .2 Et 2 O was added rapidly to the stirring reaction mixture. This solution was capped and stirred at −78° C. for 60 minutes and then brought to r.t. and stirred for 15 minutes. The resulting reaction mixture was concentrated to dryness, taken up in C 6 D 6 , and filtered through Celite, giving an orange solution which was analyzed by IR and NMR. By IR and NMR, the major species appeared to be (CP iPr 3 )Fe(N 2 )(H) (9), and some (CP iPr 3 )FeN 2 (11) was also present. No uncoordinated (CP iPr 3 )H ligand could be detected.

In a separate experiment, the reaction was carried out as above, but after being allowed to stir at room temperature for 15 minutes the reaction mixture was filtered and to the filtrate was added an aliquot of a standard solution of 1,3,5-trimethoxybenzene (0.0106 mmol). The combined solution was concentrated to dryness, taken up in C 6 D 6 , filtered through Celite, and analyzed by IR and NMR. NMR integration (d1=10 sec) of the diamagnetic peaks shows approximately 70% yield of 9 relative to the starting catalyst ( FIG. 49 ).

Curves were generated by creating solutions of [NH 4 ][Cl] and [N 2 H 5 ][HSO 4 ] of known concentrations and then analyzing by the appropriate UV-Vis methodology (vide supra)

Quantification of ammonia formed without added reductant. A sample of 12[K(Et 2 O) 0.5 ] was dissolved in Et 2 O (1 mL) and cooled to −78° C. in a Schlenk tube. HBAr F 4 .2 Et 2 O (10 equiv.) was dissolved in cold Et 2 O (1 mL) and added in one portion with rapid stirring. The reaction mixture was stirred at −78° C. for 1 hour and then at room temperature for 20 minutes, and then subjected to the standard ammonia quantification procedure. One run using 0.0025 mmol of 12[K(Et 2 O) 0.5 ] gave <0.05 equiv. of NH 3 (below detection limits) while a second run using 0.005 mmol of 12[K(Et 2 O) 0.5 ] gave 0.06 equiv. of NH 3 /Fe.

Attempted catalysis with 9. (CP 3 iPr )Fe(N 2 )(H) (1.7 mg, 0.002 mmol) was suspended in Et 2 O (0.5 mL) in a small Schlenk tube equipped with a stir bar. This solution was cooled to −78° C. in a cold well inside of the glove box. A suspension of KC 8 (14 mg, 0.100 mmol) in Et 2 O (0.75 mL) was cooled to −78° C. and added to the reaction mixture with stirring. After five minutes, a similarly cooled solution of HBAr F 4 .2 Et 2 O (93 mg, 0.092 mmol) in Et 2 O (1.0 mL) was added to the suspension in one portion with rapid stirring. Any remaining acid was dissolved in cold Et 2 O (0.25 mL) and added subsequently, and the Schlenk tube was sealed. The reaction was allowed to stir for 60 minutes at −78° C. before being warmed to room temperature and stirred for 15 minutes. The reaction was subjected to the standard workup and ammonia detection procedure; no ammonia was detected.

Attempted catalysis with [(C Si P Ph 3 )FeN 2 ][K(benzo-15-crown-5) 2 ]: [(C Si P Ph 3 )FeN 2 ][K(benzo-15-crown-5) 2 ] (2.0 mg, 0.0020 mmol) was dissolved in Et 2 O (0.5 mL) in a 20 mL scintillation vial equipped with a stir bar. This dark brown solution was vigorously stirred and cooled to −78° C. in a cold well inside of the glove box. A similarly cooled solution of HBAr F 4 .2 Et 2 O (93 mg, 0.092 mmol) in Et 2 O (1.5 mL) was added to the solution in one portion with rapid stirring. Any remaining acid was dissolved in cold Et 2 O (0.25 mL) and added subsequently. The reaction mixture was allowed to stir for 5 minutes before being transferred into a pre-cooled Schlenk tube equipped with a stirbar. The original reaction vial was washed with cold Et 2 O (0.25 mL) which was subsequently transferred to the Schlenk tube. Solid KC 8 (14 mg, 0.100 mmol) was suspended in cold Et 2 O (0.75 mL) and added dropwise to the rapidly stirred solution in the Schlenk tube which was then tightly sealed. The reaction was allowed to stir for 60 minutes at −78° C. before being warmed to room temperature and stirred for 15 minutes. Ammonia was quantified via the standard method. Two trials were performed, giving 0.40 and 0.14 equiv. NH 3 /Fe.

Crystal Structure Tables and Refinement Information

One solvent molecule is present in the structure of 6, a toluene molecule which was modeled as disordered over a two-fold special position. Additionally, one isopropyl group on the ligand was refined as a disorder over two positions in a 76:24 ratio.

The structure of 8 includes one benzene molecule per asymmetric unit.

The refinement of the crystal structure of 11 used SHELX HKLF 5 refinement to treat the presence of a non-merohedral twin accounting for 20% of the observed reflections.

This crystal structure was modeled with a 12% occupancy of the chloride complex, CP 3 FeCl. Additionally, one isopropyl group of the CP 3 ligand was modeled as disordered over two positions in a 49:51 ratio, and one trifluoromethyl group of the BArF anion was modeled as disordered over two positions in a 48:52 ratio.

Tables of Selected Bond Lengths and Angles

Computational Results:

Coordinates for NBO analysis of 12[K(Et 2 O) 3 ] (from crystallographic structure): Center Atomic Atom- Num- Num- ic Coordinates (Angstroms) ber ber Type X Y Z  1 26 0 −1.103223 −0.012183 0.135795  2 19 0 4.315234 −0.139004 −0.090250  3 15 0 −1.675324 −1.173876 1.915082  4 15 0 −1.065123 −1.018414 −1.816226  5 15 0 −1.284994 2.184405 0.235987  6 8 0 4.528660 2.280807 −1.344490  7 8 0 5.911867 0.348888 1.940111  8 7 0 0.612427 −0.071693 0.411598  9 7 0 1.748732 −0.115596 0.595993  10 8 0 4.713206 −2.836955 −0.525656  11 6 0 −3.683547 −1.101439 −1.107075  12 6 0 −2.848212 2.944956 2.477923  13 1 0 −2.902445 3.411133 3.314657  14 1 0 −2.943217 2.002256 2.629817  15 1 0 −3.549324 3.247396 1.896562  16 6 0 −5.300719 0.467098 1.314827  17 1 0 −5.754021 0.820458 0.583473  18 6 0 −4.001912 −0.590995 3.509773  19 1 0 −3.561669 −0.948234 4.247172  20 6 0 −3.352643 −0.565059 2.266253  21 6 0 −5.340093 −2.690118 −1.938757  22 1 0 −6.213047 −3.009821 −1.927115  23 6 0 −3.239989 0.059982 −0.210069  24 6 0 −5.939548 0.424855 2.554070  25 1 0 −6.807180 0.746825 2.637851  26 6 0 −3.995744 −0.009489 1.140999  27 6 0 −4.738827 1.530724 −1.725792  28 1 0 −5.272600 0.789814 −1.900313  29 6 0 −2.833970 2.529034 −0.662244  30 6 0 −0.699925 −0.964724 3.508975  31 1 0 −1.255246 −1.274126 4.255707  32 6 0 −3.177781 3.758444 −1.223575  33 1 0 −2.652914 4.506767 −1.050375  34 6 0 −5.064227 2.756173 −2.308949  35 1 0 −5.796246 2.817735 −2.878437  36 6 0 −1.997936 −3.044284 2.072403  37 1 0 −1.172254 −3.499981 1.807364  38 6 0 −3.630193 1.387339 −0.885512  39 6 0 −4.982113 −1.629900 −1.110692  40 1 0 −5.621718 −1.261735 −0.545281  41 6 0 −5.290076 −0.092274 3.658747  42 1 0 −5.709168 −0.107013 4.489935  43 6 0 −3.084558 −3.496396 1.100751  44 1 0 −3.186989 −4.449759 1.155643  45 1 0 −2.836970 −3.251424 0.205987  46 1 0 −3.914011 −3.072883 1.330188  47 6 0 −2.374639 −3.573776 3.467980  48 1 0 −3.208651 −3.185989 3.743407  49 1 0 −1.689325 −3.336150 4.096275  50 1 0 −2.461494 −4.529577 3.434330  51 6 0 −3.115644 −2.771435 −2.799728  52 1 0 −2.483847 −3.154087 −3.366548  53 6 0 −0.750968 −1.148162 −4.740497  54 1 0 −1.580024 −1.620407 −4.840874  55 1 0 −0.038712 −1.776973 −4.607887  56 1 0 −0.578243 −0.631117 −5.530824  57 6 0 1.502649 −2.065440 −2.315231  58 1 0 1.815103 −1.592208 −1.539649  59 1 0 1.549957 −1.489740 −3.081720  60 1 0 2.053171 −2.837709 −2.460536  61 6 0 −2.747844 −1.699193 −1.980183  62 6 0 −0.325574 0.487045 3.757235  63 1 0 0.179496 0.553496 4.571506  64 1 0 −1.124499 1.014869 3.832211  65 1 0 0.203405 0.811036 3.024982  66 6 0 −0.022549 −3.492168 −0.929395  67 1 0 0.581790 −4.220856 −1.087339  68 1 0 −0.916832 −3.829177 −0.851393  69 1 0 0.221701 −3.043661 −0.116590  70 6 0 −1.487758 3.224594 1.824152  71 1 0 −0.808956 2.902562 2.452414  72 6 0 0.050193 −2.507521 −2.095661  73 1 0 −0.255203 −2.969370 −2.904599  74 6 0 0.580691 −1.821703 3.461949  75 1 0 1.123702 −1.546872 2.719257  76 1 0 0.343951 −2.746055 3.359005  77 1 0 1.073156 −1.706308 4.278167  78 6 0 −0.838007 −0.212835 −3.526978  79 1 0 0.005710 0.283396 −3.493229  80 6 0 5.552067 0.484633 3.317571  81 1 0 5.929145 1.302376 3.676051  82 1 0 5.908135 −0.261935 3.825577  83 6 0 7.315816 0.488607 1.718843  84 1 0 7.801006 −0.186282 2.219940  85 1 0 7.613666 1.363018 2.015963  86 6 0 −1.299822 4.739239 1.722486  87 1 0 −1.914962 5.096379 1.078526  88 1 0 −0.400500 4.932966 1.446202  89 1 0 −1.464279 5.140526 2.579276  90 6 0 −4.406596 −3.272724 −2.778249  91 1 0 −4.643071 −3.990150 −3.321219  92 6 0 −0.039374 3.319208 −0.609736  93 1 0 −0.364441 4.240727 −0.526365  94 6 0 5.211785 3.482161 −0.977128  95 1 0 4.717915 4.251132 −1.301884  96 1 0 6.092941 3.493459 −1.381554  97 6 0 5.431388 −3.446517 −1.561156  98 1 0 4.909694 −4.171249 −1.941774  99 1 0 6.257089 −3.817878 −1.214349 100 6 0 1.307915 3.224629 0.107122 101 1 0 1.676736 2.348031 −0.020042 102 1 0 1.184523 3.386169 1.045478 103 1 0 1.909864 3.882193 −0.252694 104 6 0 4.068393 0.517784 3.442582 105 1 0 3.721485 1.265472 2.948807 106 1 0 3.827230 0.606058 4.366826 107 1 0 3.699349 −0.296154 3.090825 108 6 0 7.581088 0.327433 0.244385 109 1 0 7.263162 −0.529819 −0.046117 110 1 0 8.524717 0.391569 0.080971 111 1 0 7.123341 1.019454 −0.239674 112 6 0 −1.943089 0.817273 −3.782952 113 1 0 −1.701239 1.365393 −4.533243 114 1 0 −2.051741 1.370207 −3.005635 115 1 0 −2.768224 0.362999 −3.969832 116 6 0 0.135799 3.014067 −2.094347 117 1 0 0.816189 3.583836 −2.459410 118 1 0 −0.692505 3.169693 −2.553507 119 1 0 0.395576 2.095831 −2.203060 120 6 0 5.338946 3.557421 0.519011 121 1 0 4.464517 3.536775 0.915276 122 1 0 5.781619 4.374392 0.762325 123 1 0 5.851605 2.809564 0.835438 124 6 0 4.081567 2.325066 −2.694760 125 1 0 4.843965 2.383314 −3.292513 126 1 0 3.527624 3.108279 −2.831847 127 6 0 −4.305306 3.877919 −2.045170 128 1 0 −4.540148 4.701034 −2.407612 129 6 0 5.733516 −2.440262 −2.609574 130 1 0 4.912970 −2.089325 −2.961565 131 1 0 6.234523 −2.854135 −3.317077 132 1 0 6.248687 −1.726357 −2.227458 133 6 0 3.297851 1.090761 −2.994716 134 1 0 3.881903 0.327903 −2.972567 135 1 0 2.902512 1.166548 −3.865794 136 1 0 2.606298 0.983340 −2.337685 137 6 0 3.297906 −3.191439 1.321519 138 1 0 3.844961 −2.661537 1.906929 139 1 0 2.836541 −3.860832 1.835131 140 1 0 2.656223 −2.626817 0.883823 141 6 0 4.136732 −3.836024 0.327386 142 1 0 4.839780 −4.339808 0.765526 143 1 0 3.606577 −4.453089 −0.201928

Natural Population and Charge Analysis

Summary of Natural Population Analysis: Atom No Charge Core Valence Rydberg Total Fe  1 −1.46741  17.98394  9.44097 0.04249 27.46741  K  2 0.64775 17.99326  0.35849 0.00050 18.35225  P  3 1.27701 9.99593 3.65507 0.07199 13.72299  P  4 1.27393 9.99590 3.65705 0.07311 13.72607  P  5 1.26960 9.99597 3.66199 0.07244 13.73040  O  6 −0.62206  1.99980 6.60170 0.02056 8.62206 O  7 −0.62871  1.99981 6.61012 0.01878 8.62871 N  8 0.13757 1.99930 4.78375 0.07939 6.86243 N  9 −0.38115  1.99962 5.34359 0.03794 7.38115 O  10 −0.61888  1.99980 6.59853 0.02055 8.61888 C  11 0.03324 1.99900 3.94356 0.02420 5.96676 C  12 −0.58933  1.99939 4.58194 0.00799 6.58933 H  13 0.20074 0.00000 0.79796 0.00130 0.79926 H  14 0.21689 0.00000 0.78114 0.00197 0.78311 H  15 0.21007 0.00000 0.78860 0.00132 0.78993 C  16 −0.20245  1.99889 4.18565 0.01790 6.20245 H  17 0.19715 0.00000 0.80047 0.00238 0.80285 C  18 −0.19551  1.99919 4.18152 0.01480 6.19551 H  19 0.18943 0.00000 0.80850 0.00208 0.81057 C  20 −0.36151  1.99893 4.33351 0.02906 6.36151 C  21 −0.19506  1.99933 4.18256 0.01317 6.19506 H  22 0.19161 0.00000 0.80668 0.00171 0.80839 C  23 −0.24383  1.99889 4.20581 0.03913 6.24383 C  24 −0.19708  1.99933 4.18432 0.01343 6.19708 H  25 0.19149 0.00000 0.80686 0.00166 0.80851 C  26 0.03137 1.99899 3.94618 0.02346 5.96863 C  27 −0.21664  1.99913 4.20149 0.01602 6.21664 H  28 0.19763 0.00000 0.80004 0.00233 0.80237 C  29 −0.36260  1.99893 4.33489 0.02879 6.36260 C  30 −0.53199  1.99929 4.51377 0.01892 6.53199 H  31 0.20509 0.00000 0.79210 0.00281 0.79491 C  32 −0.17988  1.99894 4.16487 0.01608 6.17988 H  33 0.18683 0.00000 0.81093 0.00223 0.81317 C  34 −0.19064  1.99931 4.17808 0.01325 6.19064 H  35 0.19093 0.00000 0.80734 0.00173 0.80907 C  36 −0.52730  1.99926 4.51037 0.01768 6.52730 H  37 0.20537 0.00000 0.79253 0.00210 0.79463 C  38 0.03888 1.99897 3.93768 0.02447 5.96112 C  39 −0.20266  1.99890 4.18569 0.01808 6.20266 H  40 0.19621 0.00000 0.80127 0.00252 0.80379 C  41 −0.21185  1.99931 4.19886 0.01368 6.21185 H  42 0.19221 0.00000 0.80600 0.00180 0.80779 C  43 −0.58951  1.99940 4.58192 0.00819 6.58951 H  44 0.20163 0.00000 0.79705 0.00133 0.79837 H  45 0.21463 0.00000 0.78340 0.00198 0.78537 H  46 0.21065 0.00000 0.78794 0.00140 0.78935 C  47 −0.60288  1.99940 4.59489 0.00858 6.60288 H  48 0.20842 0.00000 0.78997 0.00161 0.79158 H  49 0.19819 0.00000 0.80052 0.00129 0.80181 H  50 0.20729 0.00000 0.79144 0.00127 0.79271 C  51 −0.18450  1.99894 4.16936 0.01620 6.18450 H  52 0.18808 0.00000 0.80963 0.00229 0.81192 C  53 −0.60314  1.99941 4.59520 0.00854 6.60314 H  54 0.20821 0.00000 0.79017 0.00162 0.79179 H  55 0.19684 0.00000 0.80188 0.00127 0.80316 H  56 0.20700 0.00000 0.79174 0.00126 0.79300 C  57 −0.59043  1.99941 4.58309 0.00792 6.59043 H  58 0.20584 0.00000 0.79087 0.00329 0.79416 H  59 0.20060 0.00000 0.79807 0.00134 0.79940 H  60 0.19874 0.00000 0.79959 0.00167 0.80126 C  61 −0.36574  1.99894 4.33721 0.02958 6.36574 C  62 −0.59173  1.99941 4.58456 0.00776 6.59173 H  63 0.19847 0.00000 0.80017 0.00135 0.80153 H  64 0.21141 0.00000 0.78727 0.00132 0.78859 H  65 0.21350 0.00000 0.78420 0.00230 0.78650 C  66 −0.58995  1.99940 4.58279 0.00775 6.58995 H  67 0.19510 0.00000 0.80351 0.00140 0.80490 H  68 0.21447 0.00000 0.78424 0.00129 0.78553 H  69 0.21245 0.00000 0.78575 0.00180 0.78755 C  70 −0.52833  1.99925 4.51129 0.01779 6.52833 H  71 0.20886 0.00000 0.78893 0.00221 0.79114 C  72 −0.53421  1.99929 4.51541 0.01950 6.53421 H  73 0.20462 0.00000 0.79260 0.00279 0.79538 C  74 −0.59659  1.99940 4.58966 0.00753 6.59659 H  75 0.21373 0.00000 0.78432 0.00195 0.78627 H  76 0.20192 0.00000 0.79687 0.00121 0.79808 H  77 0.20178 0.00000 0.79689 0.00133 0.79822 C  78 −0.53038  1.99926 4.51316 0.01796 6.53038 H  79 0.20219 0.00000 0.79557 0.00224 0.79781 C  80 −0.04176  1.99917 4.03024 0.01235 6.04176 H  81 0.17980 0.00000 0.81822 0.00198 0.82020 H  82 0.18048 0.00000 0.81756 0.00195 0.81952 C  83 −0.04055  1.99918 4.02912 0.01225 6.04055 H  84 0.18398 0.00000 0.81407 0.00195 0.81602 H  85 0.18176 0.00000 0.81610 0.00214 0.81824 C  86 −0.60429  1.99941 4.59648 0.00840 6.60429 H  87 0.20382 0.00000 0.79463 0.00155 0.79618 H  88 0.19918 0.00000 0.79960 0.00122 0.80082 H  89 0.20902 0.00000 0.78966 0.00131 0.79098 C  90 −0.21908  1.99933 4.20583 0.01391 6.21908 H  91 0.19181 0.00000 0.80641 0.00178 0.80819 C  92 −0.53192  1.99929 4.51319 0.01944 6.53192 H  93 0.20485 0.00000 0.79225 0.00291 0.79515 C  94 −0.04050  1.99917 4.02887 0.01245 6.04050 H  95 0.18419 0.00000 0.81373 0.00208 0.81581 H  96 0.17602 0.00000 0.82199 0.00198 0.82398 C  97 −0.04141  1.99921 4.02880 0.01339 6.04141 H  98 0.18298 0.00000 0.81479 0.00223 0.81702 H  99 0.17782 0.00000 0.82014 0.00204 0.82218 C 100 −0.59222  1.99940 4.58489 0.00793 6.59222 H 101 0.21333 0.00000 0.78348 0.00319 0.78667 H 102 0.20373 0.00000 0.79499 0.00127 0.79627 H 103 0.19554 0.00000 0.80292 0.00154 0.80446 C 104 −0.61128  1.99945 4.60495 0.00688 6.61128 H 105 0.21744 0.00000 0.78076 0.00180 0.78256 H 106 0.22131 0.00000 0.77775 0.00094 0.77869 H 107 0.21682 0.00000 0.78151 0.00168 0.78318 C 108 −0.61590  1.99944 4.60973 0.00673 6.61590 H 109 0.20825 0.00000 0.79001 0.00173 0.79175 H 110 0.22330 0.00000 0.77583 0.00088 0.77670 H 111 0.20956 0.00000 0.78844 0.00200 0.79044 C 112 −0.58856  1.99939 4.58096 0.00821 6.58856 H 113 0.20075 0.00000 0.79802 0.00124 0.79925 H 114 0.21848 0.00000 0.77936 0.00215 0.78152 H 115 0.20985 0.00000 0.78888 0.00127 0.79015 C 116 −0.59188  1.99940 4.58480 0.00768 6.59188 H 117 0.19375 0.00000 0.80484 0.00141 0.80625 H 118 0.21584 0.00000 0.78280 0.00135 0.78416 H 119 0.20764 0.00000 0.79076 0.00160 0.79236 C 120 −0.61677  1.99943 4.61060 0.00674 6.61677 H 121 0.22032 0.00000 0.77814 0.00154 0.77968 H 122 0.21640 0.00000 0.78267 0.00092 0.78360 H 123 0.20455 0.00000 0.79327 0.00218 0.79545 C 124 −0.04114  1.99918 4.02940 0.01256 6.04114 H 125 0.17691 0.00000 0.82114 0.00195 0.82309 H 126 0.18667 0.00000 0.81131 0.00202 0.81333 C 127 −0.21780  1.99933 4.20443 0.01404 6.21780 H 128 0.19017 0.00000 0.80811 0.00172 0.80983 C 129 −0.61559  1.99945 4.60971 0.00643 6.61559 H 130 0.21392 0.00000 0.78443 0.00165 0.78608 H 131 0.22119 0.00000 0.77791 0.00090 0.77881 H 132 0.20221 0.00000 0.79625 0.00154 0.79779 C 133 −0.61285  1.99945 4.60617 0.00723 6.61285 H 134 0.19711 0.00000 0.80130 0.00159 0.80289 H 135 0.22545 0.00000 0.77350 0.00105 0.77455 H 136 0.22507 0.00000 0.77309 0.00183 0.77493 C 137 −0.61910  1.99947 4.61242 0.00721 6.61910 H 138 0.20434 0.00000 0.79398 0.00168 0.79566 H 139 0.22460 0.00000 0.77445 0.00095 0.77540 H 140 0.22981 0.00000 0.76837 0.00182 0.77019 C 141 −0.04266  1.99914 4.03108 0.01244 6.04266 H 142 0.17873 0.00000 0.81929 0.00198 0.82127 H 143 0.18493 0.00000 0.81298 0.00209 0.81507

Natural Bond Orbital Analysis for Bonds of Interest

(Occupancy) Bond orbital/Coefficients/Hybrids 1. (1.84620) BD (1)Fe1—P3 (25.00%) 0.5000*Fe1 s(22.10%)p 2.97(65.54%)d 0.56(12.36%) f 0.00(0.01%) 0.0000 0.0000 0.0018 −0.4700 0.0030 0.0000 −0.0006 −0.1440 0.0154 0.0016 −0.0079 0.0000 0.0003 −0.4789 0.0132 −0.0005 −0.0004 0.0000 0.0014 0.6363 −0.0072 0.0047 0.0020 0.0503 0.0003 −0.0007 −0.1079 −0.0087 0.0016 −0.1041 0.0224 −0.0069 −0.2673 −0.0003 −0.0053 0.1624 −0.0056 0.0040 −0.0011 0.0011 −0.0075 −0.0013 −0.0018 −0.0005 −0.0016 (75.00%) 0.8660* P3 s(45.61%)p 1.19(54.37%)d 0.00(0.02%) 0.0000 0.0009 0.6748 0.0261 0.0044 −0.0013 0.0004 0.2963 −0.0194 0.0014 0.0002 0.2694 −0.0281 0.0033 −0.0003 −0.6165 0.0463 0.0044 0.0013 −0.0073 −0.0067 −0.0023 0.0082 2. (1.84783) BD (1)Fe1—P4 (24.66%) 0.4966*Fe1 s(22.28%)p 2.95(65.73%) d 0.54(11.98%) f 0.00(0.01%) 0.0000 0.0000 0.0012 −0.4720 0.0029 0.0000 −0.0004 0.0875 0.0133 0.0038 −0.0055 0.0000 −0.0013 −0.3277 0.0023 −0.0026 −0.0029 0.0000 −0.0005 −0.7360 0.0174 0.0047 −0.0025 0.0104 0.0088 −0.0022 −0.0817 0.0103 −0.0062 0.0726 −0.0142 0.0036 −0.2421 −0.0118 −0.0018 0.2199 −0.0116 0.0064 −0.0014 0.0006 −0.0072 −0.0012 0.0020 −0.0015 −0.0022 (75.34%) 0.8680* P4 s(46.11%)p 1.17(53.87%)d 0.00(0.02%) 0.0000 0.0008 0.6785 0.0271 0.0038 −0.0017 0.0003 0.1088 −0.0037 0.0030 0.0002 0.4019 −0.0288 −0.0074 0.0004 0.6016 −0.0503 0.0011 0.0019 0.0006 0.0097 −0.0040 0.0062 3. (1.84905) BD (1)Fe1—P5 (24.92%) 0.4992*Fe1 s(22.39%)p 2.91(65.15%)d 0.56(12.45%) f 0.00(0.01%) 0.0000 0.0000 0.0018 −0.4732 0.0013 0.0000 −0.0002 −0.0073 0.0149 0.0020 −0.0063 0.0000 0.0010 0.8020 −0.0182 0.0024 0.0032 0.0000 −0.0012 0.0872 −0.0052 −0.0036 −0.0012 0.0023 −0.0101 0.0045 −0.0803 −0.0030 −0.0003 0.0345 −0.0077 0.0035 −0.3346 0.0087 −0.0083 0.0652 0.0176 −0.0019 −0.0011 0.0003 −0.0076 −0.0021 0.0012 −0.0013 −0.0024 (75.08%) 0.8665* P5 s(45.74%)p 1.19(54.24%)d 0.00(0.02%) 0.0000 0.0008 0.6758 0.0266 0.0043 −0.0014 0.0004 0.1620 −0.0073 0.0026 −0.0004 −0.7133 0.0572 0.0020 0.0000 0.0627 0.0009 −0.0065 −0.0034 0.0000 −0.0020 −0.0112 −0.0053 4. (1.93106) BD (1)Fe1—N8 (20.42%) 0.4519*Fe1 s(18.42%)p 2.68(49.34%)d 1.75(32.18%) f 0.00(0.06%) 0.0000 0.0000 0.0014 −0.4286 −0.0217 0.0000 −0.0003 0.6896 0.0616 −0.0002 −0.0021 0.0000 0.0000 −0.0237 −0.0021 0.0002 0.0016 0.0000 −0.0001 0.1161 0.0098 0.0012 −0.0010 −0.0345 −0.0021 −0.0003 0.1546 0.0061 0.0013 −0.0024 −0.0013 0.0002 0.4791 0.0198 0.0040 −0.2581 −0.0108 −0.0023 −0.0051 −0.0130 −0.0009 0.0078 −0.0001 0.0185 −0.0022 (79.58%) 0.8921* N8 s(64.45%)p 0.55(35.55%)d 0.00(0.00%) 0.0000 0.8016 0.0435 −0.0028 −0.0001 −0.5864 0.0458 0.0048 0.0212 −0.0016 −0.0002 −0.0950 0.0043 0.0016 −0.0001 0.0004 0.0000 0.0019 −0.0009 5. (1.76371) BD (1)Fe1—C23 (27.45%) 0.5240*Fe1 s(14.70%)p 3.43(50.36%)d 2.38(34.92%) f 0.00(0.03%) 0.0000 0.0000 0.0024 −0.3833 0.0068 0.0000 −0.0017 −0.6994 −0.0290 0.0077 −0.0040 0.0000 0.0001 0.0268 0.0008 −0.0002 −0.0014 0.0000 −0.0002 −0.1130 −0.0047 0.0025 0.0005 −0.0362 0.0015 −0.0002 0.1614 −0.0070 0.0025 −0.0047 −0.0002 −0.0003 0.4992 −0.0195 0.0072 −0.2682 0.0103 −0.0036 0.0020 0.0085 −0.0020 −0.0059 0.0005 −0.0120 0.0006 (72.55%) 0.8517* C23 s(26.97%)p 2.71(73.03%)d 0.00(0.00%) 0.0007 0.5192 0.0058 0.0073 0.0001 0.8427 −0.0354 −0.0055 −0.0320 0.0006 0.0004 0.1334 −0.0063 −0.0012 0.0004 −0.0009 −0.0001 −0.0024 0.0012 1513. (0.04357) BD*(1)Fe1—P3 (75.00%) 0.8660*Fe1 s(22.10%)p 2.97(65.54%)d 0.56(12.36%) f 0.00(0.01%) 0.0000 0.0000 0.0018 −0.4700 0.0030 0.0000 −0.0006 −0.1440 0.0154 0.0016 −0.0079 0.0000 0.0003 −0.4789 0.0132 −0.0005 −0.0004 0.0000 0.0014 0.6363 −0.0072 0.0047 0.0020 0.0503 0.0003 −0.0007 −0.1079 −0.0087 0.0016 −0.1041 0.0224 −0.0069 −0.2673 −0.0003 −0.0053 0.1624 −0.0056 0.0040 −0.0011 0.0011 −0.0075 −0.0013 0.0018 −0.0005 −0.0016 (25.00%) −0.5000* P3 s(45.61%)p 1.19(54.37%)d 0.00(0.02%) 0.0000 0.0009 0.6748 0.0261 0.0044 −0.0013 0.0004 0.2963 −0.0194 0.0014 0.0002 0.2694 −0.0281 0.0033 −0.0003 −0.6165 0.0463 0.0044 0.0013 −0.0073 −0.0067 −0.0023 0.0082 1514. (0.04400) BD*(1)Fe1—P4 (75.34%) 0.8680*Fe 1 s(22.28%)p 2.95(65.73%)d 0.54(11.98%) f 0.00(0.01%) 0.0000 0.0000 0.0012 −0.4720 0.0029 0.0000 −0.0004 0.0875 0.0133 0.0038 −0.0055 0.0000 −0.0013 −0.3277 0.0023 −0.0026 −0.0029 0.0000 −0.0005 −0.7360 0.0174 0.0047 −0.0025 0.0104 0.0088 −0.0022 −0.0817 0.0103 −0.0062 0.0726 −0.0142 0.0036 −0.2421 −0.0118 −0.0018 0.2199 −0.0116 0.0064 −0.0014 0.0006 −0.0072 −0.0012 0.0020 −0.0015 −0.0022 (24.66%) −0.4966* P 4 s(46.11%)p 1.17(53.87%)d 0.00(0.02%) 0.0000 0.0008 0.6785 0.0271 0.0038 −0.0017 0.0003 0.1088 −0.0037 0.0030 0.0002 0.4019 −0.0288 −0.0074 0.0004 0.6016 −0.0503 0.0011 0.0019 0.0006 0.0097 −0.0040 0.0062 1515. (0.04386) BD*(1)Fe 1 - P 5 (75.08%) 0.8665*Fe1 s(22.39%)p 2.91(65.15%)d 0.56(12.45%) f 0.00(0.01%) 0.0000 0.0000 0.0018 −0.4732 0.0013 0.0000 −0.0002 −0.0073 0.0149 0.0020 −0.0063 0.0000 0.0010 0.8020 −0.0182 0.0024 0.0032 0.0000 −0.0012 0.0872 −0.0052 −0.0036 −0.0012 0.0023 −0.0101 0.0045 −0.0803 −0.0030 −0.0003 0.0345 −0.0077 0.0035 −0.3346 0.0087 −0.0083 0.0652 0.0176 −0.0019 −0.0011 0.0003 −0.0076 −0.0021 0.0012 −0.0013 −0.0024 (24.92%) −0.4992* P5 s(45.74%)p 1.19(54.24%)d 0.00(0.02%) 0.0000 0.0008 0.6758 0.0266 0.0043 −0.0014 0.0004 0.1620 −0.0073 0.0026 −0.0004 −0.7133 0.0572 0.0020 0.0000 0.0627 0.0009 −0.0065 −0.0034 0.0000 −0.0020 −0.0112 −0.0053 1516. (0.08300) BD*(1)Fe1—N8 (79.58%) 0.8921*Fe1 s(18.42%)p 2.68(49.34%)d 1.75(32.18%) f 0.00(0.06%) 0.0000 0.0000 0.0014 −0.4286 −0.0217 0.0000 −0.0003 0.6896 0.0616 −0.0002 −0.0021 0.0000 0.0000 −0.0237 −0.0021 0.0002 0.0016 0.0000 −0.0001 0.1161 0.0098 0.0012 −0.0010 −0.0345 −0.0021 −0.0003 0.1546 0.0061 0.0013 −0.0024 −0.0013 0.0002 0.4791 0.0198 0.0040 −0.2581 −0.0108 −0.0023 −0.0051 −0.0130 −0.0009 0.0078 −0.0001 0.0185 −0.0022 (20.42%) −0.4519* N8 s(64.45%)p 0.55(35.55%)d 0.00(0.00%) 0.0000 0.8016 0.0435 −0.0028 −0.0001 −0.5864 0.0458 0.0048 0.0212 −0.0016 −0.0002 −0.0950 0.0043 0.0016 −0.0001 0.0004 0.0000 0.0019 −0.0009 1517. (0.09062) BD*(1)Fe1—C23 (72.55%) 0.8517*Fe1 s(14.70%)p 3.43(50.36%)d 2.38(34.92%) f 0.00(0.03%) 0.0000 0.0000 0.0024 −0.3833 0.0068 0.0000 −0.0017 −0.6994 −0.0290 0.0077 −0.0040 0.0000 0.0001 0.0268 0.0008 −0.0002 −0.0014 0.0000 −0.0002 −0.1130 −0.0047 0.0025 0.0005 −0.0362 0.0015 −0.0002 0.1614 −0.0070 0.0025 −0.0047 −0.0002 −0.0003 0.4992 −0.0195 0.0072 −0.2682 0.0103 −0.0036 0.0020 0.0085 −0.0020 −0.0059 0.0005 −0.0120 0.0006 (27.45%) −0.5240* C23 s(26.97%)p 2.71(73.03%)d 0.00(0.00%) 0.0007 0.5192 0.0058 0.0073 0.0001 0.8427 −0.0354 −0.0055 −0.0320 0.0006 0.0004 0.1334 −0.0063 −0.0012 0.0004 −0.0009 −0.0001 −0.0024 0.0012 (0.03482) BD*(1) C11—C23 (48.70%) 0.6978* C11 s(33.58%)p 1.98(66.38%)d 0.00(0.03%) −0.0001 −0.5792 −0.0172 −0.0008 0.0009 −0.2099 0.0082 −0.0022 −0.6201 0.0243 0.0038 −0.4842 0.0106 0.0047 −0.0066 −0.0035 −0.0148 0.0080 0.0001 (51.30%) −0.7163* C23 s(24.80%)p 3.03(75.13%)d 0.00(0.07%) 0.0002 −0.4979 0.0073 0.0026 0.0001 0.2526 0.0037 −0.0023 0.6602 0.0177 0.0084 0.5012 0.0072 0.0070 −0.0160 −0.0119 −0.0162 0.0080 −0.0005 1537. (0.03133) BD*(1) C11—C39 (49.58%) 0.7041* C11 s(33.03%)p 2.03(66.94%)d 0.00(0.02%) 0.0002 −0.5746 0.0136 0.0025 −0.0007 0.7661 0.0163 0.0115 0.2861 −0.0003 −0.0004 −0.0117 −0.0095 0.0016 −0.0089 −0.0019 0.0018 −0.0092 0.0076 (50.42%) −0.7101* C39 s(33.98%)p 1.94(65.94%)d 0.00(0.08%) −0.0002 −0.5829 −0.0049 −0.0014 −0.7523 0.0055 0.0184 −0.3044 −0.0110 0.0071 0.0055 −0.0137 −0.0018 −0.0173 0.0018 −0.0100 −0.0141 0.0134 1538. (0.31948) BD*(2) C11—C39 (53.31%) 0.7301* C11 s(0.01%)p 1.00(99.94%)d 0.00(0.05%) 0.0000 −0.0091 −0.0008 0.0005 0.0025 −0.2612 0.0062 −0.0044 0.6523 −0.0040 0.0005 −0.7110 0.0039 −0.0054 −0.0060 0.0132 0.0050 0.0137 0.0093 (46.69%) −0.6833* C39 s(0.00%)p 1.00(99.97%)d 0.00(0.02%) −0.0001 −0.0048 0.0023 0.0001 −0.2638 0.0066 0.0017 0.6482 −0.0144 −0.0055 −0.7138 0.0129 0.0081 0.0090 −0.0090 −0.0039 −0.0043 0.0067 1558. (0.03658) BD*(1) C23—C26 (51.30%) 0.7162* C23 s(23.91%)p 3.18(76.01%)d 0.00(0.07%) −0.0003 0.4890 −0.0055 −0.0027 −0.0003 −0.4236 −0.0066 0.0002 −0.0470 0.0005 −0.0019 0.7603 0.0164 0.0102 0.0010 −0.0242 −0.0001 0.0069 0.0093 (48.70%) −0.6978* C26 s(32.91%)p 2.04(67.05%)d 0.00(0.03%) 0.0002 0.5734 0.0183 0.0001 −0.0007 0.3768 −0.0128 0.0010 0.0404 0.0037 −0.0022 −0.7253 0.0264 0.0054 −0.0006 −0.0125 −0.0013 0.0037 0.0127 1559. (0.03587) BD*(1) C23—C38 (51.26%) 0.7160* C23 s(24.27%)p 3.12(75.66%)d 0.00(0.07%) −0.0003 0.4926 −0.0058 −0.0027 0.0002 −0.2136 −0.0013 0.0027 0.7478 0.0151 0.0099 −0.3889 −0.0133 −0.0023 −0.0168 0.0086 −0.0151 −0.0112 −0.0028 (48.74%) −0.6981* C38 s(33.14%)p 2.02(66.83%)d 0.00(0.03%) 0.0002 0.5754 0.0159 0.0008 −0.0006 0.1860 −0.0064 0.0016 −0.7162 0.0203 0.0055 0.3463 −0.0171 −0.0001 −0.0052 0.0044 −0.0119 −0.0114 −0.0037

Second-Order Perturbation Analysis: Donor-Acceptor Interactions Involving the Fe—C a Bond:

1 Kliegl, A. Ber. Deutsch. Chem. Ges. 1908, 40, 4937-42.

2 Watt, G. W.; Chrisp, J. D. Anal. Chem. 1952, 24, 2006.

Conversion of Fe—NH 2 to Fe—N 2 with Release of NH 3

Tris(phosphine)borane ligated Fe(I) centers featuring N 2 H 4 , NH 3 , NH 2 , and OH ligands are described herein. The conversion of Fe—NH 2 to Fe—NH 3 + by addition of acid, and subsequent reductive release of NH 3 to generate Fe—N 2 , is demonstrated. This sequence models the final steps of proposed Fe-mediated nitrogen fixation pathways. The five-coordinate trigonal bipyramidal complexes described are unusual in that they adopt S=3/2 ground states and are prepared from a four-coordinate, S=3/2 trigonal pyramidal precursor.

Due the structural and mechanistic complexity of biological nitrogen fixation 1 a variety of mechanisms have been proposed that invoke either Mo or Fe as the likely active site for N 2 binding and reduction. Fe—NH 2 is an intermediate common to both limiting mechanisms (i.e., distal vs. alternating) being considered for Fe-mediated N 2 fixation scenarios at the FeMo-cofactor. 2,3 Such a species may form either via reductive protonation of the nitride intermediate of a distal scheme (i.e., Fe(N)→Fe(NH)→Fe(NH 2 )→Fe(NH 3 )), or by reductive protonation of a hydrazine intermediate of an alternating scheme (i.e., Fe(NH 2 —NH 2 )→Fe(NH 2 )+NH 3 ). In the latter context, detection of an EPR active Fe—NH 2 or possibly Fe—NH 3 common intermediate has been proposed under reducing conditions at the FeMo-cofactor from substrates including N 2 , N 2 H 4 , and MeN═NH. 3a

One key to realizing a catalytic cycle in either limiting scenario concerns the regeneration of Fe—N 2 from Fe—NH 2 with concurrent release of NH 3 . 4 While there have been recent synthetic reports demonstrating NH 3 generation from Fe(N) nitride model complexes, these studies have not provided information about the plausible downstream Fe(NH x ) (X=1, 2, 3) intermediates en route to NH 3 release, nor have these systems illustrated the feasibility of regeneration of Fe—N 2 . 5 Herein we describe a terminal, S=3/2 Fe—NH 2 complex for which the stepwise conversion to Fe—NH 3 , and then to Fe—N 2 along with concomitant release of NH 3 , is demonstrated (eqns 1 and 2).

Fe—NH 2 +H + →Fe—NH 3 +   (1)

Fe—NH 3 + +e − +N 2 →Fe—N 2 +NH 3   (2)

Addition of methyllithium to (TPB)FeBr 6 affords the corresponding methyl complex (TPB)FeMe (1) in high yield (Scheme 1). Protonation of 1 by HBAr F 4 .2Et 2 O (BAr F 4 − ═B(3,5-C 6 H 3 (CF 3 ) 2 ) 4 − ) in a cold ethereal solution releases methane to yield [(TPB)Fe][BAr F 4 ] (2) which serves as a useful synthon with a vacant coordination site.

XRD data were obtained for 1 and 2 ( FIG. 52 ). The geometry of 1 is pseudo trigonal bipyramidal about Fe with an Fe—C bond length of 2.083(10) Å and an Fe—B bond length of 2.522 (2) Å. In the solid state 2 possesses a four-coordinate distorted trigonal pyramidal geometry with no close contacts in the apical site trans to boron, making this complex coordinatively unsaturated. Additionally, there is one wide P—Fe—P angle of 136°. The origin of this large angle is not clear, but suggests increased backbonding from a relatively electron rich Fe center into the phosphine ligands that would arise from this distortion.

The Fe—B distance in 2 (2.217(2) Å) is markedly shorter than that in (TPB)FeBr (2.459(5) Å), which is noteworthy because one might expect the loss of an anionic σ-donor ligand to reduce the Lewis basicity of the metal and thus weaken the Fe—B bond. For example, the Au—B distance in (TPB)AuCl (2.318 Å) lengthens upon chloride abstraction to 2.448 Å in [(TPB)Au] + . 7 To explain this difference we note that the boron center in four-coordinate 2 is less pyramidalized (Σ(C—B—C)=347.3°) than that in five-coordinate (TPB)FeBr (Σ(C—B—C)=341.2°), pointing to a weak interaction despite the short distance. These observations suggest that the geometry of 2 might be best understood as derived from a planar three-coordinate Fe(I) center distorted towards a T-shaped geometry, 8 the unusually short Fe—B distance being due largely to the constraints imposed by the ligand cage structure. This interpretation is consistent with a computational model study: the DFT (B3LYP/6-31 G(d)) optimized geometry of the hypothetical complex [(Me 2 PhP) 3 Fe] + exhibits a planar geometry with P—Fe—P angles of 134.8°, 113.1°, and 111.7°, very close to those measured for [(TPB)Fe] + (137.5°, 113.2°, 109.1°).

When considering the bonding of the (Fe—B) 7 subunit of 2 to estimate the best oxidation state and valence assignment, two limiting scenarios present themselves: Fe(III)/B(I) and Fe(I)/B(III). The structural data and computations for 2 are suggestive of a weak Fe—B interaction and indicate that this species is better understood to be Fe(I)/B(III) rather than Fe(III)/B(I). Calculations indicate that a small amount of spin density resides on the B-atom of 2 (Sl) and suggest some contribution from an Fe(II)/B(II) resonance form is also relevant. The rest of the complexes 3-6 presented herein possess significantly longer, and presumably weaker, Fe—B interactions (vide infra) and are hence also better classified as Fe(I) species. Additional spectroscopic studies (e.g., XAS and Mössbauer) will help to better map the Fe—B bonding interaction across the variable Fe—B distances and also the spin states of the complexes. These studies would thereby help to determine the value and limitation of classically derived oxidation/valence assignments for boratranes of these types. 9

Solutions of 2 are orange in Et 2 O and pale yellow-green in THF. Titration of THF into an ethereal solution of 2 results in a distinct change in the UV-vis spectrum consistent with weak THF binding. Addition of an excess of N 2 H 4 to an ethereal solution of 2 results in a slight lightening of the orange color of the solution to afford [(TPB)Fe(N 2 H 4 )][BAr F 4 ] (3) in 89% yield. Complex 3 shows a paramagnetically shifted 1 H NMR spectrum indicative of an S=3/2 Fe center that is corroborated by a room temperature solution magnetic moment, μ eff , of 3.5 μ B . Crystals of 3 were obtained and XRD analysis ( FIG. 53A ) indicates a distorted trigonal bipyramidal geometry. The Fe—N distance of 2.205(2) Å is unusually long (2.14 Å is the average quaternary N—Fe distance in the Cambridge Structural Database) reflecting its unusual quartet spin state.

Complex 3 is stable to vacuum, but solutions decompose cleanly at room temperature over hours to form the cationic ammonia complex [(TPB)Fe(NH 3 )][BAr F 4 ], 4, which was assigned by comparison of its 1 H NMR spectrum with an independently prepared sample formed by the addition of NH 3 to the cation 2. Analysis of additional degradation products shows only NH 3 and trace H 2 (Sl). The assignment of 4 as an ammonia adduct was confirmed by XRD analysis ( FIG. 53B ). Like 3, complex 4 shows a long Fe—N distance of 2.280(3) Å in the solid state. The complexes 2, 3 and 4 are unusual by virtue of their S=3/2 spin states and underscore the utility of local 3-fold symmetry with respect to stabilizing high spin states at iron, even in the presence of strong-field phosphine ligands.

Addition of NaNH 2 to the cation 2 affords the terminal amide, (TPB)Fe—NH 2 (5) in ca. 85% non-isolated yield by 1 H NMR integration. The XRD structure of 5 ( FIG. 53C ) shows an overall geometry similar to that observed in 1, 3, and 4. Of interest is the short Fe—N distance of 1.918(3) Å by comparison to 4 (2.280(3) Å). The amide hydrogens were located in the difference map and indicate a nearly planar geometry about N (with the sum of the angles around N being 355°).

While the XRD data set of 5 is of high quality, we were concerned about the difficulty in distinguishing an Fe—NH 2 group from a potentially disordered Fe—OH moiety. We therefore independently characterized the hydroxo complex, (TPB)Fe—OH (6) (Scheme 2), which possesses a geometry similar to that observed in 5 with an Fe—B distance of 2.4438(9) Å and an Fe—O distance of 1.8916(7) Å. Despite the structural similarity between 5 and 6, different spectral signatures in both their 1 H NMR and EPR ( FIG. 54 ) spectra allow for facile distinction between them. Like 2, 3, and 4, both 5 and 6 are S=3/2.

Low-temperature EPR data ( FIG. 54 ) have been obtained on complexes 1-6. All complexes show features shifted to large g-values consistent with quartet Fe species. 10 This assignment is verified by the solution magnetic moments obtained for these complexes. Variable temperature solid-state SQUID magnetic data for complexes 2-5 (Sl) also establish quartet spin state assignments and display no evidence for spin-crossover phenomena. These data show a drop in magnetic moment in the range 50-70 K for all compounds studied. We propose that this effect is due to a large zero-field splitting in these species, which is consistent with Fe centers in related geometries. 11 Simulations with zero-field splitting of 10-20 cm −1 provide reasonable fits to the data.

Parent amide complexes of first row transition metals are rare. 12 Noteworthy precedent for related terminal M-NH 2 species includes two square planar nickel complexes 12a,d and one octahedral and diamagnetic iron complex, (dmpe) 2 Fe(H)NH 2 . 12e In addition to their different coordination numbers, geometries, and spin-states, (dmpe) 2 Fe(H)(NH 2 ) and 5 show a distinct difference at the Fe—NH 2 subunit. Six-coordinate (dmpe) 2 Fe(H)(NH 2 ) is an 18-electron species without π-donation from the amide ligand, which is pyramidalized as a result. By contrast, five-coordinate 5 accommodates π-bonding from the amide. This is borne out in its much shorter Fe—N distance (1.918(3) Å for 5 vs 2.068 Å for (dmpe) 2 Fe(H)(NH 2 )), and also its comparative planarity (the sum of the angles around N is 355° for 5 vs 325° for (dmpe) 2 Fe(H)(NH 2 )).

The terminal amide 5 was then analyzed to determine its suitability as a precursor to the N 2 complex (TPB)Fe(N 2 ) via release of NH 3 and to examine the reduction/protonation vs protonation/reduction sequences as a means of effecting overall H-atom transfer to the Fe—NH 2 unit. Attempts to carry out the one-electron reduction of 5 were not informative. For example, electrochemical studies of 5 in THF fails to show any reversible reduction waves, but the addition of harsh reductants (e.g., tBuLi) to 5 shows small amounts of (TPB)Fe(N 2 ) in the product profile. A more tractable conversion sequence utilizes protonation followed by chemical reduction. Thus, the addition of HBAr F 4 .2Et 2 O to 5 at low temperature (−35° C.) rapidly generates the cationic ammonia adduct 4. The conversion is quantitative as determined by 1 H NMR spectroscopy, and 4 can be isolated in ca. 90% yield from the solution. Subsequent exposure of 4 to one equiv of KC 8 under an atmosphere of N 2 releases NH 3 and generates the (TPB)FeN 2 complex in similarly high yield.

In summary, an unusual series of S=3/2 iron complexes featuring terminally bonded N 2 H 4 , NH 3 , NH 2 , and OH functionalities has been thoroughly characterized. These complexes are supported by a tris(phosphine)borane ligand and are best described as Fe(I) species that feature weak Fe—B bonding, though other resonance contributions to the bonding scheme warrant additional consideration. The Fe—NH 2 species faithfully models the reductive replacement of the terminal NH 2 group by N 2 with concomitant release of NH 3 , lending credence to such a pathway as mechanistically viable in Fe-mediated N 2 reduction schemes. Because spectroscopic detection of a common Fe—NH 2 or Fe—NH 3 intermediate under reductive turnover of the FeMo-cofactor has been recently proposed, 3 EPR active model complexes of the types described here should prove useful for comparative purposes.

1. (a) Einsle, O.; Tezcan, F. A.; Andrade, S. L. A.; Schmid, B.; Yoshida, M.; Howard, J. B.; Rees, D. C. Science 2002, 297, 1696-1700; (b) Spatzal, T.; Aksoyoglu, M.; Zhang, L. M.; Andrade, S. L. A.; Schleicher, E.; Weber, S.; Rees, D. C.; Einsle, O. Science 2011, 334, 940-940.

2. (a) Crossland, J. L.; Tyler, D. R. Coord. Chem. Rev. 2010, 254, 1883-1894; (b) Field, L. D.; Li, H. L.; Dalgarno, S. J.; Turner, P. Chem. Commun. 2008, 1680-1682; (c) Hazari, N. Chem. Soc. Rev. 2010, 39, 4044-4056; (d) Vela, J.; Stoian, S.; Flaschenriem, C. J.; Münck, E.; Holland, P. L. J. Am. Chem. Soc. 2004, 126, 4522-4523.

3. (a) Lukoyanov, D.; Dikanov, S. A.; Yang, Z.-Y.; Barney, B. M.; Samoilova, R. I.; Narasimhulu, K. V.; Dean, D. R.; Seefeldt, L. C.; Hoffman, B. M. J. Am. Chem. Soc. 2011, 133, 11655-11664; (b) Seefeldt, L. C.; Hoffman, B. M.; Dean, D. R. Annu. Rev. Biochem. 2009, 78, 701-722.

4. Lee, Y.; Mankad, N. P.; Peters, J. C. Nat. Chem. 2010, 2, 558-565

5. (a) Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 6252-6254; (b) Scepaniak, J. J.; Fulton, M. D.; Bontchev, R. P.; Duesler, E. N.; Kirk, M. L.; Smith, J. M. J. Am. Chem. Soc. 2008, 130, 10515-10517; (c) Scepaniak, J. J.; Vogel, C. S.; Khusniyarov, M. M.; Heinemann, F. W.; Meyer, K.; Smith, J. M. Science 2011, 331, 1049-1052; (d) Scepaniak, J. J.; Young, J. A.; Bontchev, R. P.; Smith, J. M. Angew. Chem., Int. Ed. 2009, 48, 3158-3160.

6. Moret, M.-E.; Peters, J. C. Angew. Chem., Int. Ed. 2011, 50, 2063-2067.

7. Sircoglou, M.; Bontemps, S.; Bouhadir, G.; Saffon, N.; Miqueu, K.; Gu, W.; Mercy, M.; Chen, C.-H.; Foxman, B. M.; Maron, L.; Ozerov, O. V.; Bourissou, D. J. Am. Chem. Soc. 2008, 130, 16729-16738.

8. A stable T-shaped, three-coordinate Fe(I) is known: Ingleson, M. J.; Fullmer, B. C.; Buschhorn, D. T.; Fan, H.; Pink, M.; Huffman, J. C.; Caulton, K. G. Inorg. Chem. 2008, 47, 407-409.

9. (a) Amgoune, A.; Bourissou, D. Chem. Comm. 2011, 47, 859-871. (b) Hill, A. F.; Owen, G. R.; White, A. J. P.; Williams, D. J. Angew. Chem., Int. Ed. 1999, 38, 2759-2761. (c) Pang, K.; Quan, S. M.; Parkin, G. Chem. Comm. 2006, 5015-5017.

10. Stoian, S. A.; Yu, Y.; Smith, J. M.; Holland, P. L.; Bominaar, E. L.; Munck, E. Inorg. Chem. 2005, 44, 4915-4922.

11. Harman, W. H.; Harris, T. D.; Freedman, D. E.; Fong, H.; Chang, A.; Rinehart, J. D.; Ozarowski, A.; Sougrati, M. T.; Grandjean, F.; Long, G. J.; Long, J. R.; Chang, C. J. J. Am. Chem. Soc. 2010, 132, 18115-18126.

12. (a) Adhikari, D.; Mossin, S.; Basuli, F.; Dible, B. R.; Chipara, M.; Fan, H.; Huffman, J. C.; Meyer, K.; Mindiola, D. J. Inorg. Chem. 2008, 47, 10479-10490; (b) Brady, E.; Telford, J. R.; Mitchell, G.; Lukens, W. Acta Cryst. C 1995, 51, 558-560. (c) Redshaw, C.; Wilkinson, G.; Hussain-Bates, B.; Hursthouse, M. B. J. Chem. Soc. Dalton, 1992, 1803-1811; (d) Campora, J.; Palma, P.; del Rio, D.; Conejo, M. M.; Álvarez, E. Organometallics 2004, 23, 5653-5655; (e) Fox, D. J.; Bergman, R. G. J. Am. Chem. Soc. 2003, 125, 8984-8985; (f) Sofield, C. D.; Walter, M. D.; Andersen, R. A. Acta Cryst. C 2004, 60, 465-466.

General Considerations

Unless otherwise noted, all compounds were prepared by literature procedures or purchased from commercial sources. All manipulations were carried out under a dinitrogen atmosphere by utilizing standard glovebox or schlenk techniques. Solvents were dried and de-oxygenated by an argon sparge followed by passage through an activated alumina column purchased from S.G. Waters Company. All non-halogenated solvents were tested with a standard sodium-benzophenone ketyl solution to ensure the absence of oxygen and water.

NMR measurements were obtained on Varian 300, 400, or 500 MHz spectrometers. Deuterated solvents for these measurements were obtained from Cambridge Isotope Laboratories and were dried and degassed prior to use. All 1 H spectra were referenced to residual solvent peaks and all 31 P spectra were referenced to an external H 3 PO 4 standard.

EPR X-band spectra were obtained on a Bruker EMX spectrometer with the aid of Bruker Win-EPR software suite version 3.0. The spectrometer was equipped with a rectangular cavity which operated in the TE 102 mode. Temperature control was achieved with the use of an Oxford continuous-flow helium cryostat (temperature range 3.6-300 K). All spectra were recorded at 9.37 GHz with a microwave power of 20 mW, a modulation amplitude of 4 G, and a modulation frequency of 100 kHz.

X-Ray Crystallography

Data was obtained at low temperatures on a Siemens or Bruker Platform three-circle diffractometer coupled to a Bruker-AXS Smart Apex CCD detector with graphite-monochromated Mo Kα radiation (λ=0.71073), performing φ-and ω-scans. Data for complex 4 was collected on with synchrotron radiation at the Stanford Synchrotron Radiation Laboratory (SSRL) beam line 12-2 at 17 keV using a single phi axis and recorded on a Dectris Pilatus 6M. The images were processed using XDS 1 and further workup of the data was analogous to the other datasets. All structures were solved by standard direct or Patterson methods and refined against F 2 using the SHELX program package. 2,3,4 All atoms, with the exception of hydrogens, have been anisotropically refined. The hydrogen atoms bonded to atoms of interest, namely N or O, have been located in the difference map and refined semi-freely. All other hydrogen atoms were included via a standard riding model.

In the structure of complex 1 a minor component of (TPB)FeCl was found in the difference map and modeled as disorder. Additional disorder of the BAr F 4 counterion was found in complex 4. This disorder was modeled as a rotational disorder of the CF 3 groups on one of the phenyl rings, but some of the resulting F ellipsoids still display significantly prolate shapes. We feel that the shape of these ellipsoids accurately describes the actual electron density due to the rotational disorder.

Magnetic Measurements.

Data was obtained using a Quantum Designs SQUID magnetometer running MPMSR2 software (Magnetic Property Measurement System Revision 2) at a field strength of 50000 G. Complexes were massed and then suspended in eicosane wax. Samples were then inserted into the magnetometer in plastic straws sealed under nitrogen with gelatin capsules. Loaded samples were centered within the magnetometer using the DC centering scan at 35 K. Data were acquired at 20-30 K (one data point every 2 K), and 30-300 K (one data point every 10 K). The magnetic susceptibility was adjusted for diamagnetic contributions using the constitutive corrections of Pascal's constants as well as a diamagnetic correction for the eicosane and capsule. Data workup, including simulations, was performed in the JulX software package. 5 Complex 5 displayed a lower than expected magnetic moment. NMR analysis of the sample indicated the presence of ˜15% 12-crown-4, present as a result of the protocol for generation of the complex, where it is used to aid for removal of NaBAr F 4 . Accounting for this impurity leads to a magnetic moment consistent with the other samples.

Computational Methods

Geometry optimizations were performed using the Gaussian03 package. 6 The B3LYP exchange-correlation functional was employed with a 6-31G(d) basis set. The GDIIS algorithm was used. A full frequency calculation was performed on each structure to establish true minima. A model for the initial geometry of complex 2 used the crystallographically determined coordinates as a starting point for subsequent minimization. Atoms were then stripped away from this structure to reveal a Fe(PMe 2 Ph) 3 + as the starting point to determine the theoretical structure of Fe(PMe 2 Ph) 3 + by another minimization. Structural models and orbital/spin density pictures were generated from Gaussview 03.

Synthesis of (TPB)FeMe, 1

(TPB)FeBr (0.400 g, 0.55 mmol) was dissolved in 15 mL of ether and cooled to −35° C. To this stirred solution was added a 1.6 M solution of MeLi in ether (0.620 mL, 0.99 mmol). After addition the solution was allowed to warm to room temperature and was stirred for an additional hour over which time the solution changed in color from a dark brown to a deep orange red. After this time, volatiles were removed and the remaining solids were extracted with 3 mL of benzene three times. Lyophilization of benzene resulted in a dark orange powder which was washed with 5 mL of cold pentane to yield (TPB)FeMe (0.352 g, 97%). X-ray quality crystals were grown from slow evaporation of a concentrated pentane solution of 1. 1 H NMR (C 6 D 6 , δ): 74.48 (br s), 33.25 (s), 22.52 (s), 9.31 (br s), 5.73 (s), 2.65 (s), −2.33 (br s), −2.80 (s), −7.49 (br s), −16.33 (s). UV-Vis (THF) λ max , nm (ε, M −1 cm −1 ): 840 (120). Anal. Calc. for C 37 H 57 BFeP 3 : C, 67.19; H, 8.69. Found: C, 67.26; H, 8.59. Solution magnetic moment (C 6 D 6 ): 3.9 μ B .

Synthesis of [(TPB)Fe][BAr F 4 ] 2

A dark orange solution of 1 (0.037 g, 0.06 mmol) in 5 mL of Et 2 O was cooled to −35° C. Once cooled, the solution was stirred while a similarly cooled solution of HBAr F 4 .2 Et 2 O 7 in 5 mL of Et 2 O was added dropwise over 5 min. After the addition, the solution was stirred at room temperature for an additional hour before being concentrated down to 1 mL. This solution was layered with 1 mL of pentane and cooled to −35° C. for 2 days upon which time dark orange crystals of [(TPB)Fe][BAr F 4 ] had formed (0.082 g, 97%). 1 H NMR (C 6 D 6 /THF-d 8 , δ): 32.15 (br s), 25.78 (s), 23.99 (br s), 8.93 (br s), 8.27 (s, BAr F 4 ), 4.55 (br s), 1.84 (br s), −1.24 (br s), −28.05 (s). UV-Vis (Et 2 O) λ max , nm (ε, M −1 cm −1 ): 475 (1700), 765 (800). Anal. Calc. for C 68 H 66 B 2 F 24 FeP 3 : C, 54.10; H, 4.41. Found: C, 53.93; H, 4.53.

Synthesis of [(TPB)Fe(N 2 H 4 )][BAr F 4 ], 3

2 (0.356 g, 0.24 mmol) was dissolved in 10 mL in Et 2 O and stirred. To this was added N 2 H 4 (0.076 mL, 2.36 mmol) in one portion. Upon addition, the solution lightened slightly in color to a brown-orange. The solution was allowed to stir for 15 min before the solution was concentrated to 5 mL and layered with 5 mL of pentane. After 2 days at −35° C., dark orange crystals of [(TPB)Fe(N 2 H 4 )][BAr F 4 ] had formed (0.324 g, 89%). 1 H NMR (C 6 D 6 /THF-d 8 , δ): 53.72 (br s), 28.26 (s), 25.32 (s), 20.18 (br s), 8.28 (s, BAr F 4 ) 7.67 (s, BAr F 4 ), 8.14 (br s), 7.96 (br s), 3.00 (br s), 2.67 (br s), 0.30 (br s), −26.06 (s). UV-Vis (THF) λ max , nm (ε, M −1 cm −1 ): 800 (140). Anal. Calc. for C 68 H 70 B 2 F 24 FeN 2 P 3 : C, 52.98; H, 4.58; N, 1.82. Found: C, 53.03; H, 4.63; N, 1.70. Solution magnetic moment (THF-d 8 ): 3.46 μ B .

Synthesis of [(TPB)Fe(NH 3 )][BAr F 4 ], 4

A solution of 3 (0.308 g, 0.20 mmol) in 10 mL of 1:6 THF:Benzene was rapidly stirred at RT for 12 h. After this time, the volatiles were removed in vacuo and the residue was taken up in Et 2 O, filtered, and layered with pentane before being cooled to −35°. After 2 days, dark orange-red crystals of [(TPB)Fe(NH 3 )][BAr F 4 ] had formed (0.264 g, 87%). 1 H NMR (C 6 D 6 /THF-d 8 , δ): 68.22 (br s), 28.55 (s), 24.28 (s), 17.81 (br s), 8.34 (s, BAr F 4 ), 7.68 (s, BAr F 4 ), 5.74 (br s), 3.53 (s), 2.15 (br s), 1.22 (br s), −25.48 (s). UV-Vis (THF) λ max , nm (ε, M −1 cm −1 ): 871 (50). IR (KBr, cm-1): 3381 (v[NH]) Anal. Calc. for C 68 H 69 B 2 F 24 FeNP 3 : C, 54.24; H, 4.55; N, 0.92. Found: C, 53.47; H, 4.72; N, 0.94. Solution magnetic moment (THF-d 8 ): 3.63 μ B .

Synthesis of (TPB)FeNH 2 , 5

A solution of 2 (0.300 g, 0.20 mmol) in 5 mL of Et 2 O was stirred over powdered NaNH 2 (0.077 g, 1.99 mmol) for 1.5 h at room temperature. Over this time, the solution darkened from orange to a dark brown. Volatiles were removed and the remaining residue was extracted with 40 mL of pentane to yield a pale orange solution. To this solution was added 12-crown-4 (0.070 g, 0.40 mmol) to aid in the removal of NaBAr F 4 , and solids began to precipitate. The solution was allowed to stand for 1 h before filtration. Removal of solvent for 3 h at 70° C. resulted in (TPB)FeNH 2 as a dark orange powder (0.060 g, 0.09 mmol, 46%). Crystals suitable for X-ray diffraction were grown from slow evaporation of a concentrated ethereal solution. Due to the presence of a small amount of 12-crown-4 that had similar solubility properties to the product, satisfactory combustion analysis was not obtained for 5. 1 H NMR (C 6 D 6 , δ): 91.12 (br s), 38.21 (s), 25.42 (s), 4.12 (br s), 1.55 (br s), 0.21 (br s), −3.04 (br s), −5.93 (br s), −20.19(s). UV-Vis (THF) λ max , nm (ε, M −1 cm −1 ): 700 (90), 930 (80). Solution magnetic moment (C 6 D 6 ): 4.05 μ B . We also wish to note that trace amounts (<3%) of the neutral complex (TPB)Fe(N 2 ) are typically detected by NMR spectroscopy in preparations of 5. (TPB)Fe(N 2 ) and 5 also have similar solubility properties.

Synthesis of (TPB)FeOH, 6

2 (0.80 g, 0.05 mmol) was dissolved in 5 mL of Et 2 O and stirred over NaOH (0.063 g, 1.6 mmol) at room temperature for 2 h during which the color of the solution darkened to a deep brown. Volatiles were removed from the solution and the resulting solids were extracted with pentane to yield Synthesis of (TPB)FeOH (0.027, 77%) as a brown powder. Crystals suitable for X-Ray diffraction were grown by a slow evaporation of a concentrated Et 2 O solution. 1 H NMR (C 6 D 6 , δ): 89.55 (br s), 39.07 (s), 24.70 (s), 6.71 (s), 4.08 (s), 1.55 (br s), −0.52 (br s), −6.00 (br s), −21.02 (s). UV-Vis (THF) λ max , nm (ε, M −1 cm −1 ): 870 (230), 700 (218). Anal. Calc. for C 36 H 55 BFeOP 3 : C, 65.18; H, 8.36; N, 0. Found: C, 65.15; H, 8.28; N, none found. Solution magnetic moment (C 6 D 6 ): 4.12 μ B .

Protonation of 5

A 20 mL scintillation vial was charged with 5 (0.005 g, 0.007 mmol) and HBAr F 4 .2Et 2 O (0.008 g, 0.007 mmol) and cooled to −35° C. 2 mL of similarly cooled Et 2 O was added to the mixture and the color of the solution lightened rapidly. The solution was allowed to warm to room temperature over 30 minutes before volatiles were removed to yield 4 (0.010 g, 0.006 mmol, 91%). The identity of the product was determined via 1 H NMR which was identical to that observed for 4.

Reduction of 4

A 20 mL scintillation vial was charged with 4 (0.025 g, 0.016 mmol) and KC 8 (0.0024 g, 0.018 mmol). 2 mL of Et 2 O were added and the resulting dark suspension was allowed to stir for 2 h at RT. After this time, the solution was filtered and volatiles were removed to yield (TPB)Fe(N 2 ) as a brown solid. The identity of the product was determined via 1 H NMR which was identical to the reported values for (TPB)Fe(N 2 ).

Monitored Conversion of 3 to 4

3 (0.020 g, 0.013 mmol) was dissolved in a 6:1 mixture of C 6 D 6 :THF-d 8 . The resulting solution was transferred to an NMR tube equipped with a capillary containing a solution of (TPB)FeBr in a 6:1 mixture of C 6 D 6 :THF-d 8 as an internal standard. This NMR tube was sealed with a J-Young valve and was placed into a 500 MHz spectrometer which had been preheated to 60° C. The reaction was monitored via single scans every minute for 4 hours during which time complete and clean conversion from 3 to 4 was observed. After the reaction was complete, an aliquot of the headspace was analyzed by GC for the presence of H 2 . After this, volatiles were vacuum transferred onto a solution of HCl in THF. After this, volatiles were removed and the resulting solids were diluted with water to appropriate volumes to test for the presence of NH 3 via the indophenol test, 8 or N 2 H 4 with p-dimethylaminobenzaldehyde. 9 The relative amounts of products are compiled in Table 25 of this document.

Optimized coordinates [Å] for [(TPB)Fe] + :

Fe 0.00415000 −0.16241600 −0.77675900 P −2.16349100 −1.24719000 −0.66738800 P 2.39121900 −0.83777200 −0.74771500 P −0.31345400 2.27645200 −0.74537300 C 2.09021700 −1.73435800 0.82972400 C 4.02952400 1.02631900 0.64266200 H 3.98567100 0.45071500 1.57275400 H 3.19104000 1.72365500 0.64140500 H 4.95767100 1.60939200 0.65520800 C −2.51735800 −0.85749000 1.09566800 C −2.75787900 2.84164700 0.59302900 H −2.24972100 3.20855200 1.49066600 H −2.90111300 1.76680700 0.71127400 H −3.74621500 3.31410800 0.55282800 C 0.96713000 −1.34366200 1.61726000 C 0.68267900 1.41376900 1.69824000 C −3.77585900 −1.11913600 1.66212900 H −4.53614800 −1.64204200 1.09100300 C −1.50541500 −0.21277600 1.85301900 C −1.85207300 0.20954700 3.15263700 H −1.11562400 0.74145000 3.74897600 C 0.63534800 3.23145700 −2.06275600 H 0.57728600 4.28918000 −1.78194000 C 1.88147700 2.83955400 3.29296500 H 2.40769300 2.94283300 4.23815900 C 0.69310600 −2.13340400 2.75608400 H −0.16599700 −1.88793000 3.37259400 C 4.03129200 0.10104900 −0.58072200 H 4.06604600 0.72216800 −1.48419000 C 1.38189800 1.59489600 2.90786900 H 1.54195100 0.73989400 3.56048800 B 0.03447200 −0.03107800 1.40854000 C 5.27977700 −0.79677000 −0.55921200 H 6.17024200 −0.16055800 −0.49330400 H 5.38965200 −1.41148800 −1.45674600 H 5.28761200 −1.45308000 0.31716600 C 1.72157800 3.94568000 2.45737900 H 2.12008100 4.91463800 2.74408600 C 1.05459100 3.79624400 1.24157800 H 0.95646000 4.65573100 0.58420600 C 2.77223500 −2.14130600 −2.05044100 H 3.66239200 −2.68641700 −1.71685100 C −4.07155000 −0.72325700 2.96514500 H −5.04800100 −0.93619400 3.39051300 C −3.10700900 −0.04189100 3.70690700 H −3.32936500 0.29181200 4.71691400 C 2.87573600 −2.83838200 1.19803000 H 3.71142600 −3.14386600 0.57548300 C −1.96911100 3.20280400 −0.67407900 H −2.51737400 2.81757600 −1.54509900 C 1.62859900 −3.14880400 −2.20719100 H 0.71343900 −2.65749200 −2.56047300 H 1.40294400 −3.66587600 −1.27050000 H 1.89809900 −3.90564800 −2.95299300 C 1.49436100 −3.21131000 3.13086000 H 1.25256900 −3.77768600 4.02619200 C 2.11687300 2.84858500 −2.14500700 H 2.24202800 1.80944900 −2.46700100 H 2.62947700 2.97816400 −1.18858400 H 2.61717700 3.48566700 −2.88385400 C −2.49731300 −3.09116600 −0.81402900 H −3.54555800 −3.21968300 −0.51323400 C 2.59216700 −3.56984800 2.34988300 H 3.21486600 −4.41610400 2.62556400 C −1.61976400 −3.87498400 0.17378600 H −1.85554000 −4.94310500 0.10319500 H −1.78498100 −3.56065100 1.20749500 H −0.55428900 −3.75363600 −0.04648600 C 0.53094900 2.55057600 0.86165400 C −1.86943800 4.73224900 −0.80367000 H −2.87736400 5.16043800 −0.75057800 H −1.42999600 5.06125000 −1.74899100 H −1.29175300 5.16729800 0.01900400 C −2.95949200 −0.11225400 −3.13193300 H −2.81329100 −1.04459900 −3.68700100 H −2.00927500 0.43395300 −3.13573300 H −3.68632300 0.48853300 −3.69112200 C −3.47778300 −0.36144900 −1.70257400 H −3.55367100 0.61070400 −1.20046300 C −2.34128800 −3.61958100 −2.24860700 H −1.32591600 −3.46834000 −2.62965700 H −3.03896300 −3.15266000 −2.94927700 H −2.53763200 −4.69785800 −2.26329900 C −0.04759800 3.05060900 −3.43027100 H −1.09647000 3.36299400 −3.42897900 H −0.00706500 2.00371300 −3.75748600 C 3.08823800 −1.45897600 −3.39314100 H 3.31866300 −2.21769600 −4.14966000 H 3.94741000 −0.78399700 −3.33422400 H 2.22923900 −0.88166100 −3.75845100 C −4.87834200 −0.99390800 −1.73157800 H −5.54599600 −0.36236200 −2.32962300 H −4.87513300 −1.98675200 −2.19255400 H −5.32164500 −1.07767600 −0.73669900 H 0.47189800 3.64859800 −4.18772700

Optimized Coordinates [Å] for [(Me 2 PhP) 3 Fe] + :

Fe 0.15864700 −0.18495700 −0.99760500 P −0.96396300 1.83613500 −1.46879900 P −0.45971000 −2.22271000 −0.05600500 P 2.48332300 −0.04211700 −1.31271800 C −2.26951200 −2.22907300 0.27519800 C −1.24228800 2.66879600 0.14383800 C −2.79182200 −2.06800000 1.56749900 C 4.03948300 −1.45884400 0.61288100 C −0.64552100 3.89539800 0.47185600 H −0.02266300 4.41711100 −0.24807900 C −2.04536400 2.02460300 1.10222300 C −2.25786700 2.60533100 2.35188400 H −2.89062600 2.10308200 3.07801800 C 3.22714900 −1.29680000 −2.44619800 H 4.32141000 −1.26188800 −2.43248400 C 4.38348500 −0.67980900 2.87994900 H 4.81234400 −0.82007700 3.86773800 C −4.17301300 −2.01522100 1.77110700 H −4.56274900 −1.90088900 2.77861400 C 0.31804700 −2.68760000 1.54990900 H 1.38960600 −2.83560300 1.38660000 C 4.58567000 −1.63819900 1.88602900 H 5.17639900 −2.52519600 2.09670500 C 3.63159400 0.46273300 2.59776700 H 3.47437400 1.21562800 3.36480600 C 3.08377800 0.64390800 1.32807100 H 2.50486800 1.54243900 1.12392600 C −0.21389600 −3.73662900 −1.09026300 H −0.63896300 −4.62100300 −0.60524200 C −0.85737200 4.47065600 1.72657200 H −0.39598100 5.42490200 1.96403800 C −1.66358000 3.82933600 2.66729400 H −1.83040700 4.28140300 3.64050500 C −3.16079100 −2.32427200 −0.80796000 H −2.78261000 −2.44971700 −1.82004800 C 3.28247400 1.51522500 −1.91488900 H 2.90698900 1.75077900 −2.91604500 C −5.04871600 −2.11906600 0.69010800 H −6.12191000 −2.08279500 0.85179800 C −2.66656800 1.64342000 −2.16885400 H −3.18590300 2.60610400 −2.21168900 C −4.53907500 −2.27482500 −0.60131300 H −5.21468500 −2.36443900 −1.44718900 C 3.28537900 −0.31297600 0.31747500 C −0.23933400 3.11974200 −2.57722600 H 0.77356200 3.37847300 −2.25945300 H 2.88637400 −1.08677000 −3.46523900 H 2.89836600 −2.30593900 −2.18218500 H 3.02794700 2.34344400 −1.24739500 H 4.37260400 1.42000600 −1.95477900 H 0.85730500 −3.89883800 −1.24746400 H −0.68525200 −3.61298900 −2.06920800 H −0.11632500 −3.60480000 1.96078100 H 0.20638200 −1.87762100 2.27591300 H −0.85392700 4.02578200 −2.60403500 H −0.18044400 2.70383300 −3.58796700 H −3.25191000 0.95080000 −1.55774000 H −2.59769400 1.23085800 −3.18107000 H −2.51936000 1.07248800 0.87329200 H 4.21949800 −2.21267100 −0.14752200 H −2.13022700 −1.99363600 2.42490400

Mulliken Atomic Spin Densities for 2

Sum of Mulliken spin densities=3.00000

Simulation Parameters

1 W. Kabsch, J. Appl. Cryst. 1993, 26, 795.

2 Sheldrick, G. M. Acta Cryst. 1990, A46, 467.

3 Sheldrick, G. M. Acta Cryst. 2004, A64, 112.

4 Müller, P. Crystallography Reviews 2009, 15, 57.

5 http://ewww.mpi-muelheim.mpg.de/baciloginsibill/julX_en.php

6 Gaussian 03, Revision E.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. lyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, and J. A. Pople, Gaussian, Inc., Wallingford Conn., 2004.

7 Brookhart, M.; Grant, B.; Volpe, A. F. Organometallics 1992, 11, 3920-3922.

8 Weatherburn, M. W. Anal. Chem. 1967, 39, 971-974.

9 Watt, G. W.; Chrisp, J. D. Anal. Chem. 1952, 24, 2006-2008.

These combined studies varying both the pre-catalyst and the reaction conditions for the Fe-mediated reduction of N 2 to NH 3 provide several key insights on the catalysis. Firstly, all evidence still supports the agency of a molecular species. The results are generally consistent with a molecular species as the active catalyst. Furthermore, the flexible Fe—B linkage in the TPB scaffold is empirically important for catalysis. Although other ligand scaffolds with light atoms in the axial position are viable for turnover, it is unclear whether a differing mechanism enables catalysis in this system. Additionally, turnover is unsurprisingly sensitive to the exact ligand set employed as well as the reaction conditions. While all of the conditions surveyed showed a decrease in catalytic activity relative to our original report, several variations on the reaction conditions did preserve catalytic activity, providing motivation that alternate conditions to improve the catalysis may be available. Indeed, even slight steric variation in the ligand set or in the choice of solvent seems to have a significant impact on the catalysis. Taken together, the studies presented here delineate the conditions and structures that are required for catalytic turnover.

Experimental

General Considerations. Unless otherwise noted, all compounds were purchased from commercial sources and used without further purification. Complexes 4 25 and 6 26 as well as [(TPB iPr )Fe(N 2 )][Na(12-C-4) 2 ], 21 [(DPB)Fe] 2 (μ-1,2-N 2 ), 30 MgC 14 H 10 , TPB iPr , 32 TPB Ph , 27 2-Cy 2 PC 6 H 4 Br, 36 and HBAr F 4 .2 Et 2 O 37 were prepared according to literature procedures. All manipulations were carried out under an N 2 atmosphere utilizing standard glovebox or Schlenk techniques. Solvents were dried and de-oxygenated by an argon sparge followed by passage through an activated alumina column purchased from S.G. Waters Company. Solvents for catalytic runs were additionally stirred for more than 2 hours over Na/K alloy then filtered prior to use.

IR spectra were obtained via KBr pellets on a Bio-Rad Excalibur FTS 3000 spectrometer using Varian Resolutions Pro software set at 4 cm −1 resolution. Alternately, IR spectra were obtained on a Bruker Alpha ATR spectrometer on solid samples. NMR measurements were obtained on Varian 300 MHz or 500 MHz spectrometers. Deuterated solvents for these measurements were obtained from Cambridge Isotope Laboratories and were dried and degassed prior to use. All 1 H NMR spectra were referenced to residual solvent peaks while 31 P NMR measurements were referenced to an external standard of H 3 PO 4 and 11 B NMR spectra were referenced to an external standard of Et 2 O.BF 3 . UV-Visible spectra were taken on a Cary 50 spectrometer from 1100 nm to 200 nm in the fast scan mode. Samples were prepared in a 1 cm path length quartz cuvette. All samples had a blank sample background subtraction applied.

XRD data was obtained at low temperatures on a Siemens or Bruker Platform three-circle diffractometer coupled to a Bruker-AXS Smart Apex CCD detector with graphite-monochromated Mo Kα radiation (λ=0.71073), performing φ-and ω-scans. Data for complex 4 was collected on with synchrotron radiation at the Stanford Synchrotron Radiation Laboratory (SSRL) beam line 12-2 at 17 keV using a single phi axis and recorded on a Dectris Pilatus 6M. The images were processed using XDS 38 and further workup of the data was analogous to the other datasets. All structures were solved by standard direct or Patterson methods and refined against F 2 using the SHELX program package. 39,40,41 All atoms, with the exception of hydrogens, have been anisotropically refined. The hydrogen atoms bonded to atoms of interest, namely N or O, have been located in the difference map and refined semi-freely. All other hydrogen atoms were included via a standard riding model.

Catalytic runs and NH 3 quantification were performed in a manner identically to that previously reported. 24 Ammonia quantification was also performed as previously described. 42 The only exception being that a large excess of reductant was used in Na/Hg (10% by weight) runs (0.280 g, 0.140 mmol). Procedural details and results for all catalytic runs can be found infra.

TPB Cy , 7. A schlenk tube with a stir bar was filled with a solution of 2-dicyclohexylphosphinebromobenzene (1.000 g, 2.830 mmol) in toluene (10 mL) and sealed. The schlenk tube was then hooked up to a schlenk line under a stream of N 2 and the Teflon stopper of the schlenk tube was replaced with a rubber septum. The reaction vessel was then cooled to −78° C. A 1.6 M solution of n-BuLi in hexane (1.77 mL, 2.830 mmol) was then slowly added via syringe. The resulting solution turned slightly orange while stirring for 15 min at −78° C. The solution was then warmed to ambient temperature and stirred for an additional hour before being cooled back to −78° C. At this time a 1 M solution of BCl 3 in heptane (0.92 mL, 0.920 mmol) was added to the stirring reaction vessel. The pale orange color of the solution lightened slightly upon addition of the BCl 3 . The septa was exchanged with a Teflon stopcock and the solution was allowed to stir at −78° C. for 1 hour before being warmed to room temperature and stirred for an additional 2 hours during which time a brown color developed in the solution. The mixture was then heated to 90° C. for 16 hours during which time the brown color lightened to orange and solids precipitated. Solvent was removed in vacuo and the remaining waxy solid was extracted 3× with Et 2 O (10 mL) and filtered. The remaining pale orange solution was concentrated to half volume and cooled to −35° C. for 16 hours which resulted in the formation of white crystals of the title compound (0.426 g, 0.513 mmol, 55%). 1 H NMR (C 6 D 6 , δ): 8.30 (br s, 4H), 7.46 (br s, 4H), 7.24 (br s, 2H), 7.18 (m, 2H), 1.93 (br m, 12H), 1.71 (br m, 12H), 1.63 (br s, 12H), 1.36 (br s, 8H), 1.17 (br m, 22H). 31 P{ 1 H} NMR (C 6 D 6 , δ): −2.54 (br s). 11 B{ 1 H} NMR (C 6 D 6 , δ): 25.68 (vbr s). 13 C{ 1 H} NMR (C 6 D 6 , δ): 159.69 (br s), 136.12 (br s), 131.97 (s), 127.30 (s), 35.83 (s), 31.29 (br s), 30.27 (s), 27.86 (d, J=20 Hz), 27.01 (s).

(TPB iPr )FeCl, 5. A mixture of FeCl 2 (0.087 g, 0.69 mmol), TPB iPr (0.400 g, 0.69 mmol), Fe powder (0.415 g, 7.40 mmol), and THF (20 mL) was heated to 90° C. in a sealed schlenk tube under vigorous stirring for 3 days, during which time the color of the liquid phase turned from pale yellow to brown. The solids were removed from the mixture by filtration, and the solvent was removed in vacuo. The brown residue was then triturated then extracted with pentane (200 mL) and filtered through celite to give a brown solution. Solvent evaporation in vacuo afforded the product as a greenish brown powder (0.422 g, 90%). An analytically pure sample was obtained by slow concentration of a saturated pentane solution. Crystals suitable for XRD analysis were obtained upon cooling a saturated solution of 5 in pentane to −35° C. 1 H NMR (C 6 D 6 , δ): 96.93 (br s), 35.00 (s), 23.59 (s), 9.77 (br s), 5.76 (s), 1.89 (br s), 1.62 (sh), −0.24 (s), −2.22 (br s), −22.39 (s). UV-vis (THF, nm {cm −1 M −1 }): 280 {2.0·10 4 }, 320 {sh}, 560 {sh}, 790 {150}, 960 {190}. μ eff (C 6 D 6 , Evans method, 20° C.): 4.0 μ B . Anal. calcd. for C 36 H 54 BClFeP 3 : C, 63.41; H, 7.98; found: C, 63.16; H, 7.72.

(TBP Cy )FeCl, 8. A mixture of FeCl 2 (0.076 g, 0.60 mmol), 7 (0.500 g, 0.60 mmol), Fe powder (0.333 g, 6.02 mmol), and THF (10 mL) was heated to 90° C. in a sealed schlenk tube under vigorous stirring for 3 days, during which time the color of the liquid phase turned from pale yellow to brown. The solids were removed from the mixture by filtration, and the solvent was removed in vacuo. The brown residue was then triturated then extracted with pentane (200 mL) and filtered through celite to give a brown solution. Solvent evaporation in vacuo afforded the product as a greenish brown powder (0.460 g, 83%). An analytically pure sample and crystals suitable for XRD analysis were obtained by slow concentration of a saturated pentane solution. 1 H NMR (C 6 D 6 , δ): 81.75 (br s), 36.12 (s), 25.66 (s), 17.65 (br s), 5.01 (s), 3.77 (br s), 1.85 (s), 1.56 (br s), 0.14 (s), −0.13 (s), −0.58 (s), −0.74 (sh), −1.54 (s), −1.97 (br s), −3.87 (br s), −5.92 (br s), −7.12 (br s), −23.01 (s). UV-vis (THF, nm {cm −1 M −1 }): 560 {130}, 750 {100}, 950 {150}. μ eff (C 6 D 6 , Evans method, 20° C.): 3.8 μ B , Anal. calcd. for C 54 H 78 BClFeP 3 : C, 70.33; H, 8.52; found: C, 70.45; H, 8.49.

(TPB Ph )FeCl, 9. A Schlenk tube was charged with TPB Ph (0.923 g, 1.240 mmol), FeCl 2 (0.198 g, 1.560 mmol) and Fe powder (0.176 g, 3.160 mmol) and THF (50 mL). The reaction was stirred vigorously for 3 days at 70° C., during which time the slurry turned dark brown. The mixture was filtered through celite to remove the excess iron powder and the volatiles removed in vacuo. The residual solids were triturated with toluene, slurried in CH 2 Cl 2 and filtered to collect a dark brown powder (0.841 g, 76%). Crystals suitable for X-ray analysis were grown by slow concentration of a C 6 H 6 solution. 1 H NMR (d 8 -THF, δ): 34.23, 23.92, 9.90, 7.30, 4.73, 2.31, −23.84. μ eff (d 8 -THF, Evans method, 25° C.): 4.0 μ B . We were unable to obtain satisfactory elemental analysis.

(TPB Ph )Fe, 10. Sodium (0.003 g, 0.148 mmol) and mercury (0.500 g) were stirred vigorously with C 6 H 6 (1 mL). A slurry of (TPB Ph )FeCl (0.095 g, 0.107 mmol) in C 6 H 6 (10 mL) was added and the reaction mixture stirred for 6 hours at room temperature. The resulting dark red mixture was filtered and lyophilized (0.080 g, 88%). Crystals suitable for X-ray analysis were grown by vapor diffusion of diethyl ether into a concentrated THF solution at −35° C. 1 H NMR (C 6 D 6 , δ): 8.38 (t, J=8.4 Hz, 2H), 7.94 (d, J=7.4 Hz, 1H), 7.85 (t, J=7.3 Hz, 2H), 7.59 (ddt, J=9.6, 7.3, 3.1 Hz, 4H), 7.41 (d, J=7.5 Hz, 1H), 7.31-7.17 (m, 6H), 7.09-6.79 (m, 12H), 6.73 (dd, J=4.4, 2.6 Hz, 3H), 6.60 (t, J=8.6 Hz, 2H), 6.54-6.42 (m, 2H), 6.33-6.15 (m, 2H), 6.04 (t, J=5.9 Hz, 1H), 4.42 (q, J=6.7 Hz, 1H), 4.16 (q, J=5.6 Hz, 1H), 3.89 (d, J=6.1 Hz, 1H), 3.62 (q, J=6.6 Hz, 1H). 13 C{ 1 H} NMR (C 6 D 6 , δ): 160.5 (s), 145.7 (d, J=7.0 Hz), 144.1 (s), 143.8 (s), 142.9 (s), 142.5 (s), 141.9 (s), 141.5 (s), 140.9 (s), 140.6 (s), 139.8 (s), 139.7 (s), 137.7 (s), 137.4 (s), 136.0 (d, J=10.8 Hz), 135.6 (s), 135.2 (m), 134.8 (s), 134.6 (s), 132.9 (m), 132.3 (d, J=8.7 Hz), 132.0 (d, J=7.3 Hz), 130.9 (m), 129.2 (s), 129.0 (s), 128.7 (s), 126.9 (m), 126.3 (d, J=8.7 Hz), 125.9 (s), 124.3 (s), 123.8 (d, J=6.6 Hz), 123.6 (d, J=6.5 Hz), 109.2 (d, J=15.7 Hz), 94.0 (d, J=14.3 Hz), 88.6 (s), 87.0 (s), 85.8 (s), 85.5 (m). 31 P{ 1 H} NMR (C 6 D 6 , δ): 85.65 (d, J=81.8 Hz), 69.65 (d, J=82.2 Hz), −12.92 (s). 11 B{ 1 H} (C 6 D 6 , δ): 15.9. Anal. calcd. for C 54 H 42 BFeP 3 : C, 76.26; H, 4.98. Found: C, 76.69; H, 5.59.

[(TPB Cy )Fe(N 2 )][Na(12-C-4) 2 ], 1. Sodium (0.030 g, 1.304 mmol) and mercury (1.0 g) were mixed in a vial with a stir bar. Complex 8 (0.137 g, 0.150 mmol) was dissolved in THF (10 mL) and added to the freshly prepared Na/Hg amalgam. The resulting mixture was vigorously stirred for 1 hour during which time the color of the solution changed from dark brown to a deep red. The solution was then filtered through celite and volatiles were removed in vacuo to yield a red residue. This residue was taken up in Et 2 O (10 mL) and again filtered through celite to provide a dark red solution. 12-C-4 (0.052 g, 0.297 mmol) was added and the resulting solution was allowed to sit over which time the product precipitated as red crystals. 1 H NMR (THF-d 8 , δ): 18.48 (vbr s), 12.50 (br s), 10.02 (s), 7.97 (vbr s), 7.04 (br s), 5.90 (br s), 5.51 (br s), 4.22 (br s), 3.79 (s, [Na(12-C-4) 2 ]), 2.12 (br s), 1.65 (br s), 1.50 (br s), 0.96 (vbr s), 0.56 (br s), 0.30 (br s), 0.06 (br s), −1.11 (vbr, s), −3.71 (vbr s). IR (ATR, solid): v NN =1905 cm −1 . UV-vis (THF, nm {cm −1 M −1 }): 500 {sh}, 850 {40}. μ eff (THF-d 8 , Evans method, 20° C.): 1.6 μ B . Anal. calcd. for C 70 H 110 BFeN 2 NaO 8 P 3 : C, 65.16; H, 8.59; N, 2.17; found: C, 64.89; H, 8.57; N, 2.21.

[(TPB Ph )Fe(N 2 )][Na(12-C-4) 2 ], 2. Naphthalene (0.005 g, 0.039 mmol) was weighed into a vial with Na (0.01 g, 0.435 mmol), THF (2 mL) and a stir bar. The resulting mixture was then vigorously stirred for 2 hours over which time the liquid phase became dark green. The resulting solution of NaC 10 H 8 was then filtered through a glass fiber filter paper into a −35° C. solution of 10 (0.030 g, 0.035 mmol) in THF (2 mL) which rapidly changed color from orange to dark red. The resulting solution was stirred for an additional 10 minutes before volatiles were removed. The dark red residue was washed 3× with pentane (2 mL) and then extracted into Et 2 O (10 mL) and filtered through celite. The resulting red solution was then treated with 12-C-4 (0.013 g, 0.074 mmol) and a fine red powder immediately precipitated. The mixture was allowed to stand for 1 hour before the supernatant was decanted and the resulting red solids were washed 3× with Et 2 O (2 mL) to yield the title compound as a red powder (0.029 g, 66%). 1 H NMR (THF-d 8 , δ): 11.61 (br s), 9.75 (br s), 8.66 (s), 8.42 (s), 6.96 (vbr s), 6.03 (br s), 3.66 (s, [Na(12-C-4) 2 ]), 0.35 (vbr s), −0.07 (s), −0.80 (vbr s). IR (ATR, solid): vNN=1988 cm −1 . UV-vis (THF, nm {cm −1 M −1 }): 500 {sh}. μ eff (THF-d 8 , Evans method, 20° C.): 1.7 μ B . Complex 2 is unstable to prolonged vacuum which precluded satisfactory combustion analysis.

[(DPB)Fe(N 2 )][K(Bz15-C-5) 2 ], 3. A solution of [(DPB)Fe] 2 (μ-1,2-N 2 ) (0.025 g, 0.023 mmol) and K/Hg amalgam (1 g, 1% K by weight) in THF (1 mL) was stirred at RT for 4 hr. The dark red solution was decanted and filtered through glass fiber filter paper onto solid Bz15-C-5 (0.026 g, 0.098 mmol). Solvent was removed in vacuo and the resulting solids were washed 3× with Et 2 O (1 mL) and C 6 H 6 (3×1 mL) to give dark solids of 3 (0.048 g, 0.042 mmol, 90%). Single crystals were grown by layering a THF solution with Et 2 O and pentane. 1 H NMR (THF-d 8 , δ) 14.50, 12.05, 6.86 (C), 6.73 (C), 3.85 (C), 3.72 (C), 2.80, 1.01, −2.72, −4.78. IR (ATR, thin film): v NN =1935 cm −1 . Anal. calcd. for C 58 H 81 BFeKN 2 O 10 P 2 : C, 61.43; H, 7.20; N, 2.47; found: C, 60.09; H, 7.33; N, 1.71.

[2,6-dimethylanilinium][OTf]. 2,6-dimethylaniline (0.500 g, 4.1 mmol) was dissolved in pentane (10 mL). This solution was then cooled to −35° C. before HOTf (0.619 g, 4.1 mmol) was added to the solution with stirring. Upon addition of HOTf, white precipitate formed, and the resulting suspension was allowed to warm to RT and was stirred for 1 hour. After this time, the solids were allowed to settle before the supernatant was decanted off. The solids were then dried under vacuum and subsequently washed 3× with pentane (5 mL) and 3× with Et 2 O (5 mL) to yield the title compound as a white powder (0.894 g, 3.30 mmol, 80%). 1 H NMR (10:1 CDCl 3 :DMSO-d 6 , δ) 7.06 (vbr s), 6.84 (m, 3H), 2.14 (s, 6H). 13 C NMR (10:1 CDCl 3 :DMSO-d 6 , δ) 131.6 (s), 129.1 (s), 128.2 (s), 128.0 (s), 17.5 (s). Anal. calcd. for C 9 H 12 F 3 NO 3 S: C, 39.85; H, 4.46; N, 5.16; found: C, 39.84; H, 4.37; N, 4.90.

[2,6-dimethylanilinium][BAr F 4 ].1 Et 2 O. 2,6-dimethylaniline (0.036 g, 0.296 mmol) was dissolved in Et 2 O (3 mL). To this was added a solution of HBAr F 4 .2 Et 2 O (0.300 g, 0.296 mmol) in Et 2 O (3 mL) and the resulting clear solution was allowed to stir for 1 hour. After this time, the solution was concentrated to 3 mL and layered with pentane (3 mL) and cooled to −35° C. for 3 days, over which time white crystals of the product formed (0.200 g, 0.182 mmol, 62%). 1 H NMR (10:1 CDCl 3 :DMSO-d 6 , δ) 7.65 (s, 8H), 7.5 (vbr s, 3H), 7.48 (s, 4H), 7.11 (m, 1H), 7.04 (m, 2H), 3.43 (q, J=7 Hz, 4H), 2.30 (s, 6H), 1.15 (t, J=7 Hz, 6H). 13 C NMR (10:1 CDCl 3 :DMSO-d 6 , δ) 161.5 (q), 134.5 (s), 131.1 (s), 129.4 (s), 128.6 (m), 125.7 (s), 123.0 (s), 120.2 (s), 117.3 (s), 65.6 (s), 17.5 (s), 15.0 (s). Anal. calcd. for C 44 H 34 BF 24 NO: C, 49.88; H, 3.23; N, 1.32; found: C, 49.77; H, 3.03; N, 1.24.

1 Smil, V. Enriching the Earth ; MIT Press: Cambridge Mass., 2001.

2 Schlögl, R. In Handbook of Heterogeneous Catalysis ; Wiley-VCH Verlag GmbH & Co. KGaA: 2008.

3 Eady, R. R. Chem. Rev. 1996, 96, 3013-3030.

4 Burgess, B. K.; Lowe, D. J. Chem. Rev. 1996, 96, 2983-3011.

5 Chatt, J.; Dilworth, J. R.; Richards, R. L. Chem. Rev. 1978, 78, 589-625.

6 Hidai, M.; Mizobe, Y. Chem. Rev. 1995, 95, 1115-1133.

7 Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76-78.

8 Schrock, R. R. Acc. Chem. Res. 2005, 38, 955-962.

9 Arashiba, K.; Miyake, Y.; Nishibayashi, Y. Nature Chem. 2011, 3, 120-125.

10 Saouma, C. T.; Peters, J. C. Coord. Chem. Rev. 2011, 255, 920-937.

11 Hendrich, M. P.; Gunderson, W.; Behan, R. K.; Green, M. T.; Mehn, M. P.; Betley, T. A.; Lu, C. C.; Peters, J. C. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 17107-17112.

12 Brown, S. D.; Peters, J. C. J. Am. Chem. Soc. 2005, 127, 1913-1923.

13 Lee, Y.; Mankad, N. P.; Peters, J. C. Nat Chem 2010, 2, 558-565.

14 Smith, J. M.; Lachicotte, R. J.; Pittard, K. A.; Cundari, T. R.; Lukat-Rodgers, G.; Rodgers, K. R.; Holland, P. L. J. Am. Chem. Soc. 2001, 123, 9222-9223.

15 Gilbertson, J. D.; Szymczak, N. K.; Tyler, D. R. J. Am. Chem. Soc. 2005, 127, 10184-10185.

16 Field, L. D.; Li, H. L.; Magill, A. M. Inorg. Chem. 2008, 48, 5-7.

17 Hoffman, B. M.; Dean, D. R.; Seefeldt, L. C. Acc. Chem. Res. 2009, 42, 609-619.

18 Peters, J. C.; Mehn, M. P. In Activation of Small Molecules ; Wiley-VCH Verlag GmbH & Co. KGaA: 2006, p 81-119.

19 Rodriguez, M. M.; Bill, E.; Brennessel, W. W.; Holland, P. L. Science 2011, 334, 780-783.

20 Yuki, M.; Tanaka, H.; Sasaki, K.; Miyake, Y.; Yoshizawa, K.; Nishibayashi, Y. Nature Comm. 2012, 3, 1254-1254.

21 Moret, M.-E.; Peters, J. C. Angew. Chem. Int. Ed. 2011, 50, 2063-2067.

22 Moret, M.-E.; Peters, J. C. J. Am. Chem. Soc. 2011, 133, 18118-18121.

23 Anderson, J. S.; Moret, M.-E.; Peters, J. C. J. Am. Chem. Soc. 2013, 135, 534-537.

24 Anderson, J. S.; Rittle, J.; Peters, J. C.; Nature, 2013, In Press.

25 Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 10782-10783.

26 Gilbert-Wilson, R.; Field, L. D.; Colbran, S. B.; Bhadbhade, M. M. Inorg. Chem. 2013, 52, 3043-3053.

27 Kameo, H; Hashimoto, Y.; Nakazawa, H. Organometallics 2012, 31, 3155-3162

28 Tolman, C. A. Chem. Rev. 1977, 77, 313-348.

29 Bontemps, S.; Gornitzka, H.; Bouhadir, G.; Miqueu, K.; Bourissou, D. Angew. Chem. Int. Ed. 2006, 45, 1611-1614.

30 Suess, D. L. M.; Peters, J. C. J. Am. Chem. Soc. 2013, 135, 4938-4941.

31 Suess, D. L. M.; Peters, J. C. J. Am. Chem. Soc. 2013.

32 Bontemps, S.; Bouhadir, G.; Dyer, P. W.; Miqueu, K.; Bourissou, D. Inorg. Chem. 2007, 46, 5149-5151.

33 S. E. Creutz, J. C. Peters, J. Am. Chem. Soc. 2013, 136, 1105-1115.

34 Chrystiuk, E.; Williams, A. J. Am. Chem. Soc. 1987, 109, 3040-3046.

35 Beale, R. N. J. Chem. Soc. 1954, 4494-4499.

36 Murata, M.; Buchwald, S. L. Tetrahedron 2004, 60, 7397-7403.

37 Brookhart, M.; Grant, B.; Volpe, A. F. Organometallics 1992, 11, 3920-3922.

38 Kabsch, W. J. Appl. Cryst. 1993, 26, 795.

39 Sheldrick, G. M. Acta Cryst. 1990, A46, 467.

40 Sheldrick, G. M. Acta Cryst. 2004, A64, 112.

41 Müller, P. Crystallography Reviews 2009, 15, 57.

42 Weatherburn, M. W. Anal. Chem. 1967, 39, 971-974.

Comparison of Molecular Co and Fe Complexes in the Catalytic Conversion of N 2 to NH 3

Abstract

The synthesis of a series of tris(phosphino)borane-ligated Co complexes is reported. The reactivity of these species with proton and electron sources in the presence of N 2 at −78° C. is described, and notably [(TPB)Co(N 2 )][Na(12-crown-4) 2 ] (TPB=[o-( i Pr 2 P)C 6 H4] 3 B) is found to mediate the formation of 2.4 equivalents of NH 3 /Co under these conditions.

It is well understood that the ability of a molecular system to activate bound dinitrogen (N 2 ) relates to the propensity of that system to mediate functionalization of the N 2 moiety. This principle is evidenced, for example, in the early reports that Mo(0) and W(0) N 2 complexes (v (N-N) 2025-1910 cm −1 ) generate NH 3 and N 2 H 4 upon treatment with acid, 1 whereas for instance HCo(N 2 )(PPh 3 ) 3 (one of the earliest reported transition metal dinitrogen complexes, 2 v (N-N) 2088 cm −1 ) only quantitatively releases N 2 upon treatment with acid, with no evidence for N 2 functionalization. 1d,3 Furthermore, this cobalt complex can be reductively deprotonated to generate more activated Co—N 2 complexes, such as [(PPh 3 ) 3 Co(N 2 )][Li(Et 2 O) 3 ] (v (N-N) 1900 cm −1 ), which upon treatment with acid do produce some NH 3 and N 2 H 4 (in this case 0.21 and 0.22 equivalents respectively). 3 With this principle in mind extensive efforts have been made to study the activation and functionalization of N 2 bound to metal centers of varying electronic properties. 4 Indeed in some extreme cases systems have been shown to activate bound N 2 to the extent that the N—N bond is fully cleaved. 5,6 Furthermore, in many cases it is has been shown that treatment of strongly activated N 2 complexes with acid or H 2 leads to some reduced nitrogen products. 1,3,6 However, this guiding principle alone has been insufficient to lead to many synthetic species capable of catalytic conversion of N 2 to NH 3 . In this regard it is prudent to study the few systems known to catalyze this reaction with an emphasis on identifying important properties aside from their degree of bound N 2 activation. 7-9 Understanding these principles may lead to the design of better synthetic catalysts as well as providing clues as to how nitrogenase enzymes might operate.

For instance, it has recently been discovered that a tris(phosphino)borane-ligated Fe complex is capable of catalyzing the conversion of N 2 to NH 3 at −78° C. 8 It has been further postulated that the success of this system in activating N 2 stoichiometrically and mediating its catalytic conversion to NH 3 may arise from important geometric variability at the metal center provided by a highly flexible Fe—B interaction. 10,11 Such geometric variability is proposed to allow a single Fe center to access different coordination environments, alternately stabilizing π-acidic or π-basic nitrogenous moieties that may be sampled along a N 2 fixation pathway. 6 Consistent with this hypothesis, the isostructural (P 3 X)-ligated Fe systems have been studied and found to have a drastic dependence of activity on the identity of the X atom: with the least flexible X=Si system furnishing divergently low NH 3 yields, whereas the more flexible X=C or B systems afford moderate yields of NH 3 . 8,9 However, the X=Si precursor also displays weaker N 2 activation than the X=C or B species (vide infra), allowing for an alternate hypothesis whereby the importance of the M-X interaction is to modulate the π-basicity of the metal center.

In an attempt to differentiate these hypotheses, we sought to explore the activity of a species displaying comparably high metal-ligand flexibility but weak N 2 activation. Our previous studies have shown that tris(phosphino)borane-ligated Co complexes possess a flexible Co—B interaction. 12 In addition, they only weakly activate N 2 and were therefore excellent candidates as a model platform for this study. For comparison with the isostructural (TPB)Fe series whose N 2 fixation activity we have described, 8 we decided to target an analogous redox series of N 2 complexes with Co supported by the TPB ligand.

The (TPB)Co(N 2 ) complex (Scheme 1, 1) provided a logical entry point to the desired redox series. The cyclic voltammagram of 1 in THF displays a broad feature corresponding to an oxidation process at 0.2 V vs. Fc/Fc + and a quasi-reversible reduction wave at −2.0 V vs. Fc/Fc + . Prompted by this result, we explored the one electron chemical reduction and oxidation of 1. Treatment of 1 with 1 equivalent of NaC 10 H 8 followed by 2 equivalents of 12-crown-4 generates diamagnetic [Na(12-crown-4) 2 )][(TPB)Co(N 2 )] as red crystals (Scheme 1, 2).

The v (N-N) stretch of 2 is lower in energy than that of 1, 1978 and 2089 cm −1 respectively, and the solid state structure of 2 displays contracted Co—N, Co—B, and Co—P distances compared to 1, consistent with increased backbonding to each of these atoms. The one-electron oxidation of 1 can be achieved by addition of 1 equivalent of [H.(OEt 2 ) 2 ][BAr F 4 ] at low temperature followed by warming, which generates red-purple [(TPB)Co][BAr F 4 ] (Scheme 1, 3, BAr F 4 =tetrakis(3,5-bistrifluoromethlyphenyl)borate). SQUID magnetometry measurements indicate that 3 adopts a high spin (S=1) state in the solid state with no evidence for spin crossover. The structure of 3 confirms that [(TPB)Co][BAr F 4 ] does not bind N 2 in the solid state. The lack of dinitrogen binding at room temperature for 3 is consistent with the behavior of the isostructural Fe complex, [(TPB)Fe][BAr 4 F ]. 14

An assessment of the degree of Co—B bond flexibility of the (TPB)Co scaffold can be made by comparing the metal-boron interatomic distances of the halide species (TPB)CoBr and the N 2 anion 2, an analogous comparison can be made for the isostructural (TPB)Fe complexes. In the (TPB)Fe series, the Fe—B distance decreases upon reduction and N 2 coordination from 2.458(5) Å in (TPB)FeBr to 2.293(3) Å in [Na(12-crown-4) 2 )][(TPB)Fe(N 2 )] (Δ M-B =0.165 Å). 11 In the (TPB)Co series, the Co—B distance also decreases upon reduction and N 2 coordination from 2.4629(8) Å in (TPB)CoBr 12 to 2.301(3) Å in 2 (Δ M-B =0.162 Å). This evidence suggests that the (TPB)Co platform exhibits a similar degree of axial ligand flexibility to that observed for the (TPB)Fe system. Furthermore, the N 2 ligand of 2 is less activated, as evidenced by vibrational spectroscopy, than the analogous (TPB)Fe, (CP 3 )Fe, or (SiP 3 )Fe complexes whose relative N 2 fixation activity we have described ( FIG. 1 , SiP 3 =[o-( i Pr 2 P)C 6 H 4 ] 3 Si, CP 3 =[o-( i Pr 2 P)C 6 H 4 ] 3 C). This is as expected for the generally less π-basic metal center, attributable to the less spatially diffuse d-orbitals of Co.

The reactivity of these (TPB)Co complexes with sources of protons and electrons in the presence of N 2 was investigated. In analogy to the [Na(12-crown-4) 2 ][(TPB)Fe(N 2 )] complex, treatment of a suspension of 2 in Et 2 O at −78° C. with excess [H.(OEt 2 ) 2 ][BAr F 4 ] followed by excess potassium graphite (KC 8 ) under an atmosphere of N 2 leads to the formation of 2.4±0.3 equivalents of NH 3 (120% per Co, average of 6 iterations, Equation 1). No hydrazine is observed. It is worth noting that each of the 6 experimental iterations under the standard conditions provided yields of ≧2.1 equivalents of NH 3 per Co and the most productive iteration yielded 2.8 equivalents of NH 3 per Co. These nominally superstoichiometric yields are consistent with modest catalytic N 2 conversion by a subpopulation of Co complexes. Notably, no ammonia is formed when either 2, [H.(OEt 2 ) 2 ][BAr F 4 ], or KC 8 is omitted from the standard conditions, indicating that all three components are necessary for NH 3 production. These results, combined with our previous studies of N 2 fixation by (P 3 X)M complexes, demonstrate that the degree of N 2 activation by the (SiP 3 )Fe catalytic precursor is not solely responsible for the divergently low N 2 conversion activity of that system ( FIG. 83 ). Indeed, [Na(12-crown-4) 2 ] [(SiP 3 )Fe(N 2 )] shows a higher degree of N 2 activation than 2, yet 2 demonstrates higher N 2 conversion activity.

Interestingly, though anionic 2 and cationic 3 both generated substantial NH 3 under the standard conditions, submitting neutral 1 to these conditions provided attenuated yields of NH 3 , comparable to the yields obtained with (TPB)CoBr (Table 35, A-D). The effect of varying the X atom in the (P 3 X) scaffold complexes of Co was also investigated. In this context, the known X=Si complex 15 (Table 35, E) and a previously unreported X=N Co complex 16 (Table 35, F) were subjected to the standard reaction conditions. Of the (P 3 X)Co complexes screened, only (TPB)-ligated Co complexes generated ≧0.5 equivalents of NH 3 per metal center, underscoring again the importance of the nature of the M-X interaction in facilitating N 2 fixation by complexes of this type. To further explore the generality of the observed reactivity for Co complexes, additional Co precursors were submitted to the standard reaction conditions. The precursors examined consisted of a bis(phosphino)borane-ligated Co—N 2 complex 17 (Table 35, G), a bis(phosphino)boryl Co—N 2 complex 18 (H), as well as various simple Co complexes (I-K). Again none of the precursors screened provided ≧0.5 equivalents of NH 3 per metal center.

To conclude, we have described a series of tris(phosphine)borane-ligated Co complexes and demonstrated the ability of a molecular Co-dinitrogen complex to facilitate the conversion of N 2 to NH 3 at −78° C. in the presence of proton and electron sources (2.4 equivalents of NH 3 generated per Co center on average). Importantly, the propensity of the (P 3 X)M complexes we have studied to perform productive nitrogen fixation appears not to depend solely on the ability of the precursor complex to activate bound N 2 . This observation provides further support for the hypothesis that the ability of these complexes to mediate the conversion of N 2 to NH 3 arises from important geometric variability at the metal center provided by a flexible M-X interaction. This report also represents the first example of superstoichiometric N 2 to NH 3 conversion by a Co-dinitrogen complex.

(1) (a) Chatt, J.; Pearman, A. J.; Richards, R. L. Nature 1975, 253, 39-40. (b) Chatt, J.; Dilworth, J. R.; Richards, R. L. Chem. Rev. 1978, 78, 589-625. (c) Chatt, J.; Pearman, A. J.; Richards, R. L. J. Chem. Soc. Dalton Trans. 1977, 1852. (d) Hidai, M.; Takahashi, T.; Yokotake, I.; Uchida, Y. Chem. Lett. 1980, 645-646. (e) Nishihara, H.; Mori, T.; Saito, T.; Sasaki, Y. Chem. Lett. 1980, 667-668.

(2) (a) Yamamoto, A.; Kitazume, S.; Pu, L. S.; Ikeda, S. Chem. Commun. 1967, 79-80. (b) Sacco, A.; Rossi, M. Chem. Commun. 1967, 316. (c) Misono, A.; Uchida, Y.; Hidai, M.; Araki, M. Chem. Commun. 1968, 1044.

(3) Yamamoto, A.; Miura, Y.; Ito, T.; Chen, H.; Iri, K.; Ozawa, F.; Miki, K.; Sei, T.; Tanaka, N.; Kasai, N. Organometallics 1983, 2, 1429-1436.

(4) (a) Bowman, A. C.; Milsmann, C.; Atienza, C. C. H.; Lobkovsky, E.; Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc. 2010, 132, 1676-1684. (b) Chirik, P. J. Dalton Trans. 2007, 16-25. (c) Crossland, J. L.; Tyler, D. R. Coord. Chem. Rev. 2010, 254, 1883-1894. (d) Ding, K.; Brennessel, W. W.; Holland, P. L. J. Am. Chem. Soc. 2009, 131, 10804-10805. (e) Duchateau, R.; Gambarotta, S.; Beydoun, N.; Bensimon, C. J. Am. Chem. Soc. 1991, 113, 8986-8988. (f) Hanna, T. E.; Lobkovsky, E.; Chirik, P. J. Organometallics 2009, 28, 4079-4088. (g) Manriquez, J. M.; Bercaw, J. E. J. Am. Chem. Soc. 1974, 6229-6230. (h) Nikiforov, G. B.; Vidyaratne, I.; Gambarotta, S.; Korobkov, I. Angew. Chem. Int. Ed. Engl. 2009, 48, 7415-7419. (i) Pickett, C. J.; Talarmin, J. Nature 1985, 317, 652-653. (j) Pool, J. A.; Lobkovsky, E.; Chirik, P. J. Nature 2004, 427, 527-530. (k) Pun, D.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2008, 130, 6047-6054. (l) Vidyaratne, I.; Crewdson, P.; Lefebvre, E.; Gambarotta, S. Inorg. Chem. 2007, 46, 8836-8842. (m) Yandulov, D. V; Schrock, R. R.; Rheingold, A. L.; Ceccarelli, C.; Davis, W. M. Inorg. Chem. 2003, 42, 796-813.

(5) (a) Clentsmith, G. K. B.; Bates, V. M. E.; Hitchcock, P. B.; Cloke, F. G. N. J. Am. Chem. Soc. 1999, 121, 10444-10445. (b) Gambarotta, S.; Scott, J. Angew. Chem. Int. Ed. Engl. 2004, 43, 5298-5308. (c) Hebden, T. J.; Schrock, R. R.; Takase, M. K.; Müller, P. Chem. Commun. 2012, 48, 1851-1853. (d) Keane, A. J.; Yonke, B. L.; Hirotsu, M.; Zavalij, P. Y.; Sita, L. R. J. Am. Chem. Soc. 2014, 136, 9906-9909. (e) Rodriguez, M. M.; Bill, E.; Brennessel, W. W.; Holland, P. L. Science 2011, 334, 780-783.

(6) For a recent review see: Jia, H.-P.; Quadrelli, E. A. Chem. Soc. Rev. 2014, 43, 547-564.

(7) (a) Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76. (b) Schrock, R. R. Angew. Chem. Int. Ed. Engl. 2008, 47, 5512-5522. (c) Arashiba, K.; Miyake, Y.; Nishibayashi, Y. Nat. Chem. 2010, 3, 120. (d) Kuriyama, S.; Arashiba, K.; Nakaji ma, K.; Tanaka, H.; Kamaru, N.; Yoshizawa, K.; Nishibayashi, Y. J. Am. Chem. Soc. 2014, DOI: 10.1021/ja5044243.

(8) Anderson, J. S.; Rittle, J.; Peters, J. C. Nature 2013, 501, 84.

(9) Creutz, S. E.; Peters, J. C. J. Am. Chem. Soc. 2014, 136, 1105.

(10) Moret, M.-E.; Peters, J. C. J. Am. Chem. Soc, 2011, 133, 18118.

(11) Moret, M.-E.; Peters, J. C. Angew. Chem. Int. Ed. 2011, 50, 2063.

(12) Suess, D. L. M.; Tsay, C.; Peters, J. C. J. Am. Chem. Soc. 2012, 134, 14158.

(13) Lee, Y.; Mankad, N. P.; Peters, J. C. Nat. Chem. 2010, 2, 558.

(14) Anderson, J. S.; Moret, M.-E.; Peters, J. C. J. Am. Chem. Soc. 2013, 135, 534.

(15) Whited, M. T.; Mankad, N. P.; Lee, Y.; Oblad, P. F.; Peters, J. C. Inorg. Chem. 2009, 48, 2507.

(16) Co complex synthesized analogously to the Fe complex reported in: MacBeth, C. E.; Harkins, S. B.; Peters, J. C. Can. J. Chem. 2005, 83, 332.

(17) Co complex synthesized analogously to the Fe complex reported in: Suess, D. L. M.; Peters, J. C. J. Am. Chem. Soc. 2013, 135, 4938

(18) Lin, T.-P.; Peters, J. C. J. Am. Chem. Soc. 2013, 135, 15310.

General Considerations:

All manipulations were carried out using standard Schlenk or glovebox techniques under an N 2 atmosphere. Solvents were deoxygenated and dried by thoroughly sparging with N 2 followed by passage through an activated alumina column in a solvent purification system by SG Water, USA LLC. Nonhalogenated solvents were tested with sodium benzophenone ketyl in tetrahydrofuran in order to confirm the absence of oxygen and water. Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc., degassed, and dried over activated 3-Å molecular sieves prior to use.

[H.OEt 2 ][BAr F 4 ], 1 KC 8 , 2 (TPB)Co(N 2 ), 3 (TPB)CoBr, 3 (SiP 3 )Co(N 2 ), 4 NArP 3 , 5 (PBP)Co(N 2 ), 6 DPB, 7 and Co(PPh 3 ) 2 | 2 8 were prepared according to literature procedures. All other reagents were purchased from commercial vendors and used without further purification unless otherwise stated. Et 2 O for NH 3 generation reactions was stirred over Na/K (≧2 hours) and filtered before use.

Physical Methods:

Elemental analyses were performed by Midwest Microlab, LLC (Indianapolis, Ind.). 1 H and 13 C chemical shifts are reported in ppm relative to tetramethylsilane, using 1 H and 13 C resonances from residual solvent as internal standards. 31 P chemical shifts are reported in ppm relative to 85% aqueous H 3 PO 4 . Solution phase magnetic measurements were performed by the method of Evans. 9 IR measurements were obtained as thin films formed by evaporation of solutions using a Bruker Alpha Platinum ATR spectrometer with OPUS software. Optical spectroscopy measurements were collected with a Cary 50 UV-vis spectrophotometer using a 1-cm two-window quartz cell. Electrochemical measurements were carried out in a glovebox under an N 2 atmosphere in a one compartment cell using a CH Instruments 600B electrochemical analyzer. A glassy carbon electrode was used as the working electrode and platinum wire was used as the auxiliary electrode. The reference electrode was Ag/AgCl in THF. The ferrocene couple (Fc + /Fc) was used as an internal reference. THF solutions of electrolyte (0.3 M tetra-n-butylammonium hexafluorophosphate, TBAPF 6 ) and analyte were also prepared under an inert atmosphere.

X-ray Crystallography:

X-ray diffraction studies were carried out at the Caltech Division of Chemistry and Chemical Engineering X-ray Crystallography Facility on a Bruker three-circle SMART diffractometer with a SMART 1K CCD detector. Data was collected at 100K using Mo Kα radiation (λ=0.71073 Å). Structures were solved by direct or Patterson methods using SHELXS and refined against F2 on all data by full-matrix least squares with SHELXL-97.20 All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed at geometrically calculated positions and refined using a riding model. The isotropic displacement parameters of all hydrogen atoms were fixed at 1.2 (1.5 for methyl groups) times the Ueq of the atoms to which they are bonded.

Notes Specific to Individual Structures:

The 12-crown-4 fragments are disordered for [Na(12-crown-4) 2 ][(TPB)Co(N 2 )]. One 12-crown-4 (with O atoms labeled O5-O8 and O50-O80) is fully disordered over two unique positions. The other 12-crown-4 (O atoms labeled O1-O4) shows a disorder in the methyl carbons but not the oxygen atoms. In all cases, the positions of the carbons could be located in the difference map and refined anisotropically and the hydrogen atoms were placed at geometrically calculated positions as usual. The oxygen atom in one of the solvent THF molecules (O10 and O100) is disordered over two positions. The other THF molecule (with O9) shows large thermal ellipsoids, potentially indicating an unresolved disorder of this moiety. A diethylether molecule was located on an inversion center and is therefore disordered about this symmetry element. The occupancies of all disordered fragments were freely refined and the bond lengths and angles were restrained to be the same for the disordered fragments. Hydrogen atoms were not included on any of the solvent molecules for these reasons.

(DPB)Co(N 2 ) crystallizes with two monometallic (DPB)Co(N 2 ) complexes and one half of a bimetallic [(DPB)Co(N 2 )] 2 complex in each asymmetric unit. This was also observed for the isostructural complex (DPB)Ni(N 2 ). 10 For the bimetallic fragment, one of the isopropyl substituents on P1 is disordered in which the methine carbon is disordered over two positions (C7A and C7B) and one of the methyl carbons is disordered over two positions (C8A and C8B). The occupancies of the disordered fragments were freely refined and the bond lengths and angles were restrained to be the same for the disordered fragments. As such, hydrogen atoms were not included on this isopropyl substituent.

The fluorine substituents on C143, C144, C152, and C168 in [(TPB)Co][BAr 4 F ] are disorded by rotation about the C—C bonds to varying extents and were refined as two-part positional disorders in each case. The occupancies of the disordered fragments were freely refined and the bond lengths and angles were restrained to be the same for the disordered fragments.

[Na(12-crown-4) 2 ][(TPB)Co(N 2 )] (2): To a −78° C. solution of (TPB)CoBr (70.5 mg, 0.0967 mmol) in THF (2 mL) was added a freshly prepared solution of NaC 10 H 8 (23.5 mg C 10 H 8 , 0.222 mmol) in THF (3 mL). The solution was brought to RT and allowed to stir for six hours. Addition of 12-crown-4 (51.1 mg, 0.290 mmol) and removal of solvent in vacuo provided a dark red solid. Et 2 O was added and subsequently removed in vacuo. The residue was suspended in C 6 H 6 and filtered and the solids were washed with C 6 H 6 (2×2 mL) and pentane (2×2 mL) to furnish a red solid (68.8 mg, 0.0660 mmol, 68%). Single crystals were grown by vapor diffusion of pentane onto a THF solution of the title compound that had been layered with Et 2 O. NMR peaks are somewhat broadened likely owing to the presence of a small amount of (TPB)Co(N 2 ). 1 H NMR (400 MHz, THF-d 8 ) δ 7.41 (3H), 6.94 (3H), 6.66 (3H), 6.44 (3H), 3.64 (32H), 2.29 (br), 1.37 (6H), 1.20 (6H), 0.93 (6H), −0.26 (6H). 11 B NMR (128 MHz, THF-d 8 ) δ 9.32. 31 P NMR (162 MHz, THF-d 8 ) δ 62.03. IR (thin film, cm −1 ): 1978 (N 2 ). Anal. Calcd. for C 52 H 86 BCoN 2 NaO 8 P 3 : C, 59.32; H, 8.23; N, 2.66. Found: C, 59.05; H, 7.99; N, 2.47.

[(TPB)Co][BAr F 4 ] (3): To a −78° C. solution of (TPB)Co(N 2 ) (91.5 mg, 0.135 mmol) in Et 2 O (2 mL) was added solid [H(OEt 2 ) 2 ][BAr F 4 ] (134.0 mg, 0.132 mmol). The reaction was brought to RT and vented to allow for the escape of H 2 . The purple-brown solution was stirred for 1 hr. The solution was layered with pentane (5 mL) and stored at −35° C. to furnish red-purple single crystals of the title compound (162.9 mg, 0.0952 mmol, 82%) which were washed with pentane (3×2 mL). 1 H NMR (400 MHz, C 6 D 6 ) δ 26.25, 23.80, 8.64, 8.44 ([BAr F 4 ]), 7.88 ([BAr F 4 ]), 6.33, −2.16, −3.68. UV-Vis (Et 2 O, nm {L·cm −1 ·mol −1 }): 585 {1500}, 760 {532}. Anal. Calcd. for C 68 H 66 B 2 CoF 24 P 3 : C, 53.99; H, 4.40. Found: C, 53.94; H, 4.51.

(DPB)Co(N 2 ): A solution of DPB (0.8483 g, 1.79 mmol) and CoCl 2 (0.2316 g, 1.78 mmol) in THF (ca. 80 mL) was stirred at RT until all of the solids dissolved, leaving a deep blue solution. Solvent was removed in vacuo and rated in Et 2 O for 15 min. Solvent was removed in vacuo and the blue residue was dissolved in benzene (80 mL). Freshly prepared 1% sodium mercury amalgam (0.0864 g Na, 3.76 mmol) was added to the dark blue solution and stirred vigorously for 24 hours. The resulting dark red-orange solution was decanted and filtered through celite. Lyophilization of the decanted benzene solution leaves a fine dark brown powder. This powder was dissolved in minimal Et 2 O and cooled in a freezer to −30° C. affording red-orange crystals of (2). (0.850 g, 1.51 mmol, 84.8%) Solution magnetic moment (Evans method, RT, C 6 D 6 ): 1.8 μ B . IR (thin film, cm −1 ): 2098 (N 2 ). 1 H NMR (C 6 D 6 , 300 MHz, 23° C.): δ 17.79, 14.75, 4.05, 2.40, −2.72, −4.04; UV-Vis (toluene, nm {M −1 cm −1 }): 288 {9350}. Elemental analysis shows low values for N consistent with a labile N 2 ligand Calcd. for C 52 H 86 BCoN 2 NaO 8 P 3 : C, 64.19; H, 7.36; N, 4.99. Found: C, 64.52; H, 7.57; N, 4.13.

[(NArP 3 )CoCl][BPh 4 ]: THF (5 mL) was added to a solid mixture of NArP 3 (58 mg, 91.2 mmol), CoCl 2 (12 mg, 92.4 mmol) and NaBPh 4 (31 mg, 90.6 mmol). The reaction was stirred for 4 hours at room temperature during which the color evolved from yellow to green to purple. The solvent was removed in vacuo and the residue was taken in dichloromethane. The suspension was filtered over a plug of Celite and the filtrate was dried yielding a purple powder (86 mg, 82.1 mmol, 90%). Single crystals were grown by slow evaporation of a saturated solution of [(NArP 3 )CoCl][BPh 4 ] in diethyl ether/dichloromethane (1:2 v:v). 1 H NMR (CD 2 Cl 2 , 300 MHz, 23° C.): δ 177.77, 37.50, 23.78, 13.48, 12.96, 7.37, 7.08, 6.92, 4.41, 1.50, −3.60, −9.81; UV-Vis (THF, nm {L·cm −1 ·mol −1 }): 564 {452}, 760 {532}; μ eff (CD 2 Cl 2 , Evans method, 23° C.): 3.97 μ B . Anal. Calcd. for C 63 H 80 BClCoNP 3 : C, 72.10; H, 7.68; N, 1.33. Found: C, 71.97; H, 7.76; N, 1.30.

Ammonia Quantification: A Schlenk tube was charged with HCl (3 mL of a 2.0 M solution in Et 2 O, 6 mmol). Reaction mixtures were vacuum transferred into this collection flask. Residual solid in the reaction vessel was treated with a solution of [Na][O-t-Bu] (40 mg, 0.4 mmol) in 1,2-dimethoxyethane (1 mL) and sealed. The resulting suspension was allowed to stir for 10 min before all volatiles were again vacuum transferred into the collection flask. After completion of the vacuum transfer, the flask was sealed and warmed to room temperature. Solvent was removed in vacuo, and the remaining residue was dissolved in H 2 O (1 mL). An aliquot of this solution (20 μL) was then analyzed for the presence of NH 3 (present as [NH 4 ][Cl]) by the indophenol method. 11 Quantification was performed with UV-vis spectroscopy by analyzing absorbance at 635 nm.

Standard NH 3 Generation Reaction Procedure with [(TPB)Co(N 2 )][Na(12-crown-4) 2 ] (2): [(TPB)Co(N 2 )][Na(12-crown-4) 2 ] (2.2 mg, 0.002 mmol) was suspended in Et 2 O (0.5 mL) in a 20 mL scintillation vial equipped with a stir bar. This suspension was cooled to −78° C. in a cold well inside of a N 2 glovebox. A solution of [H.(OEt 2 ) 2 ][BAr F 4 ] (95 mg, 0.094 mmol) in Et 2 O (1.5 mL) similarly cooled to −78° C. was added to this suspension in one portion with stirring. Residual acid was dissolved in cold Et 2 O (0.25 mL) and added subsequently. This mixture was allowed to stir 5 minutes at −78° C., before being transferred to a precooled Schlenk tube equipped with a stir bar. The original reaction vial was washed with cold Et 2 O (0.25 mL) which was added subsequently to the Schlenk tube. KC 8 (16 mg, 0.119 mmol) was suspended in cold Et 2 O (0.75 mL) and added to the reaction mixture over the course of 1 minute. The Schlenk tube was then sealed, and the reaction was allowed to stir for 40 min at −78° C. before being warmed to room temperature and stirred for 15 min.

Standard NH 3 Generation Reaction Procedure with (TPB)Co(N 2 ) (1):

The procedure was identical to that of the standard NH 3 generation reaction protocol with the changes noted. The precursor used was (TPB)Co(N 2 ) (1.3 mg, 0.002 mmol).

Standard NH 3 Generation Reaction Procedure with [(TPB)Co][BAr F 4 ] (3):

The procedure was identical to that of the standard NH 3 generation reaction protocol with the changes noted. The precursor used was [(TPB)Co(N 2 )][BAr F 4 ] (2.3 mg, 0.002 mmol).

Standard NH 3 Generation Reaction Procedure with (TPB)CoBr:

The procedure was identical to that of the standard NH 3 generation reaction protocol with the changes noted. The precursor used was (TPB)CoBr (1.6 mg, 0.002 mmol).

Standard NH 3 Generation Reaction Procedure with (SiP 3 )Co(N 2 ):

The procedure was identical to that of the standard NH 3 generation reaction protocol with the changes noted. The precursor used was (SiP 3 )Co(N 2 ) (1.5 mg, 0.002 mmol).

Standard NH 3 Generation Reaction Procedure with [(NArP 3 )CoCl][BPh 4 ]:

The procedure was identical to that of the standard NH 3 generation reaction protocol with the changes noted. The precursor used was [(NArP 3 )CoCl][BPh 4 ] (1.9 mg, 0.002 mmol).

Standard NH 3 Generation Reaction Procedure with (DPB)Co(N 2 ):

The procedure was identical to that of the standard NH 3 generation reaction protocol with the changes noted. The precursor used was (DPB)Co(N 2 ) (1.4 mg, 0.002 mmol).

Standard NH 3 Generation Reaction Procedure with (PBP)Co(N 2 ):

The procedure was identical to that of the standard NH 3 generation reaction protocol with the changes noted. The precursor used was (PBP)Co(N 2 ) (1.1 mg, 0.002 mmol).

Standard NH 3 Generation Reaction Procedure with Co(PPh 3 ) 2 | 2 :

The procedure was identical to that of the standard NH 3 generation reaction protocol with the changes noted. The precursor used was Co(PPh 3 ) 2 | 2 (1.8 mg, 0.002 mmol).

Standard NH 3 Generation Reaction Procedure with CoCp 2 :

The procedure was identical to that of the standard NH 3 generation reaction protocol with the changes noted. The precursor used was CoCp 2 (0.6 mg, 0.003 mmol).

Standard NH 3 Generation Reaction Procedure with Co 2 (CO) 8 :

The procedure was identical to that of the standard NH 3 generation reaction protocol with the changes noted. The precursor used was Co 2 (CO) 8 (0.4 mg, 0.001 mmol, 0.002 mmol Co) sampled as a 100 μL aliquot of a stock solution (2.0 mg Co 2 (CO) 8 in 0.5 mL Et 2 O).

(1) Brookhart, M.; Grant, B.; Volpe Jr., A. F. Organometallics 1992, 11, 3920-3922.

(2) Wietz, I. S.; Rabinovitz, M. J. J. Chem. Soc., Perkin Trans. 1993, 1, 117.

(3) Suess, D. L. M.; Tsay, C.; Peters, J. C. J. Am. Chem. Soc., 2012, 134, 14158.

(4) Whited, M. T.; Mankad, N. P.; Lee, Y.; Oblad, P. F.; Peters, J. C. Inorg. Chem., 2009, 48, 2507.

(5) MacBeth, C. E.; Harkins, S. B.; Peters, J. C. Can. J. Chem., 2005, 83, 332.

(6) Lin, T.-P.; Peters, J. C. J. Am. Chem. Soc., 2013, 135, 15310.

(7) Bontemps, S.; Gornitzka, H.; Bouhadir, G.; Miqueu, K.; Bourissou, D. Angew. Chem. Int. Ed. 2006, 45, 1611-1614.

(8) Cotton, F. A.; Faut, O. D.; Goodgame, D. M. L.; Holm, R. H. J. Am. Chem. Soc., 1961, 83, 1780.

(9) Evans, D. F. J. Chem. Soc., 1959, 2003.

(10) Harman, W. H.; Lin, T.-P.; Peters, J. C. Angew. Chem. Int. Ed. Engl. 2014, 53, 1081-1086.

(11) Weatherburn, M. W. Anal. Chem. 1967, 39, 971.

(12) Watt, G. W.; Chrisp, J. D. Anal. Chem. 1952, 24, 2006.

All references cited throughout this application, for example patent documents including issued or granted patents or equivalents; patent application publications; and non-patent literature documents or other source material; are hereby incorporated by reference herein in their entireties, as though individually incorporated by reference, to the extent each reference is at least partially not inconsistent with the disclosure in this application (for example, a reference that is partially inconsistent is incorporated by reference except for the partially inconsistent portion of the reference).

The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments, exemplary embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims. The specific embodiments provided herein are examples of useful embodiments of the present invention and it will be apparent to one skilled in the art that the present invention may be carried out using a large number of variations of the devices, device components, methods steps set forth in the present description. As will be obvious to one of skill in the art, methods and devices useful for the present methods can include a large number of optional composition and processing elements and steps.

When a group of substituents is disclosed herein, it is understood that all individual members of that group and all subgroups, including any isomers, enantiomers, and diastereomers of the group members, are disclosed separately. When a Markush group or other grouping is used herein, all individual members of the group and all combinations and subcombinations possible of the group are intended to be individually included in the disclosure. When a compound is described herein such that a particular isomer, enantiomer or diastereomer of the compound is not specified, for example, in a formula or in a chemical name, that description is intended to include each isomers and enantiomer of the compound described individual or in any combination. Additionally, unless otherwise specified, all isotopic variants of compounds disclosed herein are intended to be encompassed by the disclosure. For example, it will be understood that any one or more hydrogens in a molecule disclosed can be replaced with deuterium or tritium. Isotopic variants of a molecule are generally useful as standards in assays for the molecule and in chemical and biological research related to the molecule or its use. Methods for making such isotopic variants are known in the art. Specific names of compounds are intended to be exemplary, as it is known that one of ordinary skill in the art can name the same compounds differently.

It must be noted that as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural reference unless the context clearly dictates otherwise. Thus, for example, reference to “a cell” includes a plurality of such cells and equivalents thereof known to those skilled in the art, and so forth. As well, the terms “a” (or “an”), “one or more” and “at least one” can be used interchangeably herein. It is also to be noted that the terms “comprising”, “including”, and “having” can be used interchangeably. The expression “of any of claims XX-YY” (wherein XX and YY refer to claim numbers) is intended to provide a multiple dependent claim in the alternative form, and In an embodiment is interchangeable with the expression “as in any one of claims XX-YY.”

Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are now described. Nothing herein is to be construed as an admission that the invention is not entitled to antedate such disclosure by virtue of prior invention.

Every formulation or combination of components described or exemplified herein can be used to practice the invention, unless otherwise stated.

Whenever a range is given in the specification, for example, a temperature range, a time range, or a composition or concentration range, all intermediate ranges and subranges, as well as all individual values included in the ranges given are intended to be included in the disclosure. As used herein, ranges specifically include the values provided as endpoint values of the range. For example, a range of 1 to 100 specifically includes the end point values of 1 and 100. It will be understood that any subranges or individual values in a range or subrange that are included in the description herein can be excluded from the claims herein.

As used herein, “comprising” is synonymous with “including,” “containing,” or “characterized by,” and is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. As used herein, “consisting of” excludes any element, step, or ingredient not specified in the claim element. As used herein, “consisting essentially of” does not exclude materials or steps that do not materially affect the basic and novel characteristics of the claim. In each instance herein any of the terms “comprising”, “consisting essentially of” and “consisting of” may be replaced with either of the other two terms. The invention illustratively described herein suitably may be practiced in the absence of any element or elements, limitation or limitations which is not specifically disclosed herein.

One of ordinary skill in the art will appreciate that starting materials, biological materials, reagents, synthetic methods, purification methods, analytical methods, assay methods, and biological methods other than those specifically exemplified can be employed in the practice of the invention without resort to undue experimentation. All art-known functional equivalents, of any such materials and methods are intended to be included in this invention. The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention that in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims.